[llvm-branch-commits] [Clang][Sema] Add __builtin_infer_alloc_token() declaration and semantic checks (PR #163638)

[Marco Elver via llvm-branch-commits]

https://github.com/melver created https://github.com/llvm/llvm-project/pull/163638

This change introduces the `__builtin_infer_alloc_token()` builtin declaration
and adds the necessary semantic checks in Sema.