Fix XcodeToolchain extra flags getting overwritten across configurations

[czoido]

Changelog: Fix: XcodeToolchain's extra flags (tools.build:cflags/cxxflags/ldflags/defines) no longer get overwritten when installing more than one configuration for the same project.
Docs: TODO

Running conan install twice for the same project, once per build_type, with
tools.build:cxxflags set to a different value each time:

conan install . -s build_type=Debug   -c tools.build:cxxflags=["-DDEBUG_FLAG"]
conan install . -s build_type=Release -c tools.build:cxxflags=["-DRELEASE_FLAG"]

overwrites the generated file conan_global_flags.xcconfig entirely on the
second install. Only -DRELEASE_FLAG remains afterwards and -DDEBUG_FLAG is gone.
Both the Debug and Release configurations in the resulting Xcode project end
up using -DRELEASE_FLAG, regardless of which one is actually built. No error
or warning is produced.

Other settings XcodeToolchain writes (C++ standard, minimum OS version, etc.) don't have this problem, each configuration keeps its own value after multiple installs. Only the extra flags confs (cxxflags, cflags, ldflags, defines) are affected.