[clang-format] handle trailing comments in function definition detection

A follow-up to
https://github.com/llvm/llvm-project/commit/f6bc614546e169bb1b17a29c422ebace038e6c62
where we handle the case where the semicolon is followed by a trailing
comment.

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D107907