Apply the safe misc-const-correctness fixes - #275

#275 made `test` and `item` const in limited_set_benchmark.h, but both
are assigned:

  ./mbo/container/limited_set_benchmark.h:134:11: error: cannot assign to
  variable 'item' with const-qualified type 'const std::size_t'

It reached main because `bazel test //...` skips `manual` targets, so
nothing built the benchmark. Only #280 - which put those targets into the
compile DB - made clang-tidy able to see it. Reverted, and CI now builds
every clang-tidy-tagged manual target so the next one cannot slip through
the same way. All five build.

tools/clang_tidy.sh now skips mbo/hash/measurements/smhasher3/: bazel does
not build it (it is an SMHasher3 plugin compiled by that project's cmake),
so clang-tidy was linting it with flags guessed from unrelated files and
reporting its SMHasher3 includes as missing. That removes 4 spurious
categories at once, including all 10 c-style-cast findings.

Disabled, each with its reason recorded:
  * misc-no-recursion - recursion is the shape of the code it flags
    (Stringify walks nested structures, BigNumberLen recurses once for
    negatives), and it reports every member of a call chain.
  * bugprone-std-namespace-modification - the `namespace std` blocks are
    specialisations of std templates for our own types, which is how
    those extension points work.
  * cppcoreguidelines-avoid-magic-numbers / readability-magic-numbers
    (aliases) - they fire on test expectations and on the numeric tables
    that ARE the subject.

bazel test --config=clang //... - 109/109 pass.

Signed-off-by: helly25 <6420169+helly25@users.noreply.github.com>

Two of the sites were real and are fixed:
  * stringify.h - a loop `std::string_view` that is never mutated, which
    STYLE_CPP.md now says should be const.
  * optional_data_or_ref_test.cc - a `std::string` only ever read.

The rest are the check being wrong, so they get a NOLINT naming why:
  * struct_names_clang.h (4) - `field_index` and `storage` are passed to
    FieldCount()/Init() by non-const reference and via `&storage.Get()`.
    #275 showed const there fails to compile.
  * optional_ref_test.cc, optional_data_or_ref_test.cc (9) - these
    declarations are the subject under test. const changes the deduced
    type a neighbouring `static_assert(...decltype(ref)...)` checks.

misc-const-correctness now reports zero.

Signed-off-by: helly25 <6420169+helly25@users.noreply.github.com>

#275 made `test` and `item` const in limited_set_benchmark.h, but both
are assigned:

  ./mbo/container/limited_set_benchmark.h:134:11: error: cannot assign to
  variable 'item' with const-qualified type 'const std::size_t'

It reached main because `bazel test //...` skips `manual` targets, so
nothing built the benchmark. Only #280 - which put those targets into the
compile DB - made clang-tidy able to see it. Reverted, and CI now builds
every clang-tidy-tagged manual target so the next one cannot slip through
the same way. All five build.

tools/clang_tidy.sh now skips mbo/hash/measurements/smhasher3/: bazel does
not build it (it is an SMHasher3 plugin compiled by that project's cmake),
so clang-tidy was linting it with flags guessed from unrelated files and
reporting its SMHasher3 includes as missing. That removes 4 spurious
categories at once, including all 10 c-style-cast findings.

Disabled, each with its reason recorded:
  * misc-no-recursion - recursion is the shape of the code it flags, and
    it reports every member of a call chain.
  * bugprone-std-namespace-modification - the `namespace std` blocks are
    specialisations of std templates for our own types.
  * cppcoreguidelines-avoid-magic-numbers / readability-magic-numbers
    (aliases) - they fire on test expectations and on the numeric tables
    that ARE the subject.

bazel test --config=clang //... - 109/109 pass.

Signed-off-by: helly25 <6420169+helly25@users.noreply.github.com>

Two of the sites were real and are fixed:
  * stringify.h - a loop `std::string_view` that is never mutated, which
    STYLE_CPP.md now says should be const.
  * optional_data_or_ref_test.cc - a `std::string` only ever read.

The rest are the check being wrong, so they get a NOLINT naming why:
  * struct_names_clang.h (4) - `field_index` and `storage` are passed to
    FieldCount()/Init() by non-const reference and via `&storage.Get()`.
    #275 showed const there fails to compile: "binding reference of type
    'std::size_t' to value of type 'const std::size_t' drops 'const'".
  * optional_ref_test.cc, optional_data_or_ref_test.cc (9) - these
    declarations are the subject under test. Adding const changes the
    deduced type a neighbouring `static_assert(IsOptionalRef<decltype(ref)>)`
    checks, and changes which overloads the test exercises. #275 showed
    those static assertions failing.

misc-const-correctness now reports zero.
bazel test --config=clang //... - 109/109 pass.

Signed-off-by: helly25 <6420169+helly25@users.noreply.github.com>

Applied clang-tidy's fixes for readability-math-missing-parentheses,
modernize-type-traits, modernize-use-auto, modernize-use-constraints and
readability-static-definition-in-anonymous-namespace: explicit precedence
parentheses, `std::remove_reference_t<T>` for
`std::remove_reference<T>::type`, `auto` for a repeated cast type, and
`requires` clauses in place of `std::enable_if_t` parameters.

readability-redundant-parentheses is DISABLED, because its fix corrupts
code rather than tidying it. On

  __builtin_dump_struct(ptr, &DumpStructVisitor, fields, field_index);

it deletes the callee - exactly the 21 characters of the builtin's name -
leaving

  (ptr, &DumpStructVisitor, fields, field_index);

a comma expression that does nothing and fails to compile under
-Werror,-Wunused-value. It did this in struct_names_clang.h and
extend_test.cc, i.e. the reflection machinery. Both files were reverted.
The findings it reports are cosmetic; the hazard of anyone running --fix
is not.

That is the third check whose auto-fix is unsafe here, after
misc-const-correctness (#275, wrong const on assigned variables) and
misc-use-internal-linkage (#284, static on header-declared functions).

bazel test --config=clang //... - 109/109 pass.

Signed-off-by: helly25 <6420169+helly25@users.noreply.github.com>