[clang] [AST] Mark the fallthrough coreturn statement implicit. (PR #77465)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

Approve from my side too, but please make sure to apply suggestion from @cor3ntin and clang-format before submitting.

https://github.com/llvm/llvm-project/pull/77465