[APINotes] Introduce APINotes infrastructure in Clang Sema and Frontend

This upstreams more of the Clang API Notes functionality that is
currently implemented in the Apple fork:
https://github.com/apple/llvm-project/tree/next/clang/lib/APINotes

This adds the initial Clang APINotes infrastructure to Clang Sema and
Clang Frontend.

There will shortly be a follow-up patch with the actual usages of this
API. I'm splitting these changes into separate PRs to keep the diffs
easier to review.