[clang] Improve ABI version handling in clang

[Fznamznon]

We have a language option for ABI-compatibility. The user may choose clang release version to generate code compatible with using flag -fclang-abi-compat= . Currently to bring a new version of known ABI, at least 3 different places need to be modified:

1. Add new value to the langopt enum:
   

   llvm-project/clang/include/clang/Basic/LangOptions.h

   Line 171 in 299a55a

   enum class ClangABI {

2. Make sure it is handled here
   

   llvm-project/clang/lib/Frontend/CompilerInvocation.cpp

   Line 3925 in 299a55a

   GenerateArg(Consumer, OPT_fclang_abi_compat_EQ, "20.0");

3. Make sure it is parsed correctly here
   

   llvm-project/clang/lib/Frontend/CompilerInvocation.cpp

   Line 4479 in 299a55a

   Opts.setClangABICompat(LangOptions::ClangABI::Ver20);

So, it is at least 3 places that need to be updated every ~6 months when new release is branched. Some of the places are super easy to forget which will cause build fails or bugs like the bug resolved by #144109 several months after ClangABI::Ver20 was introduced.

It would be great to improve this.