[Clang][NFC] Introduce LanguageID::HIP_LANG and reclassify AtomicBuiltins
The TableGen class AtomicBuiltin is currently used for both OpenCL and HIP
atomic builtins, but there is no way to classify them. That class now takes
language as an argument. HIP is represented by a new enum member
LanguageID::HIP_LANG in this scheme.
Assisted-By: Claude Sonnet 4.5