[C23] Use thread_local semantics (#70107)

When implementing thread_local as a keyword in C23, we accidentally
started using C++11 thread_local semantics when using that keyword
instead of using C11 _Thread_local semantics.

This oversight is fixed by pretending the user wrote _Thread_local
instead. This doesn't have the best behavior in terms of diagnostics,
but it does correct the semantic behavior.

Fixes https://github.com/llvm/llvm-project/issues/70068
Fixes https://github.com/llvm/llvm-project/issues/69167