[clang] [clang] fix the unexpected control flow in ParseTentative.cpp (PR #109298)

[via cfe-commits]

c8ef wrote:

> I'm only worried about the missing return at the end of the function - maybe there should be a `llvm_unreachable` here to silence compilers, even if that control flow is not possible.

https://github.com/llvm/llvm-project/blob/21594f2793da5d2e1d1cd6714bfa10e742f2e526/clang/lib/Parse/ParseTentative.cpp#L914-L937

I believe we can conclude from the code snippet above that we don't actually need the `llvm_unreachable`? @tbaederr 

https://github.com/llvm/llvm-project/pull/109298