Use named constants. Regularize comparison patterns. Follow the
approach from commit b9d4db7098 (CompilerId: Clarify C and C++ standard
level detection, 2024-04-02).
In commit fc3a1cbdd8 (CompilerID: Compiler extensions default detection,
2021-05-29, v3.22.0-rc1~52^2~2), extensions were assumed to be `OFF`
for Clang targeting the MSVC ABI. However, the `clang` and `clang++`
tools both seem to have extensions enabled by default even when
targeting the MSVC ABI.
This can be observed with the `RunCMake.CompileFeatures` test.
It fails with the GNU-like `clang/clang++` front-end, but removing
the above special case makes it pass. The test passes either way
with the MSVC-like `clang-cl` front-end.
C++ style comments were added by commit fc3a1cbdd8 (CompilerID: Compiler
extensions default detection, 2021-05-29, v3.22.0-rc1~52^2~2), but they
may not be supported by the default mode of some C compilers. Use
C-style comments instead. For consistency, do this for all languages.
Fixes: #22942
