PGO builds do not generated profile data for all source files. This means that for a PGO build, you may end up with compiler warnings for source files with no profile data. Do we want to silence these warnings? If we do, it may be harder to spot if PGO builds fail to include profile data correctly.

Spotted while working on #117752.

Linked PRs

• gh-117804: Autoconf: silence some clang warnings for PGO builds #117796
• gh-117804: Document how to suppress PGO unprofiled source warnings #117859
• [3.12] gh-117804: Document how to suppress PGO unprofiled source warnings (GH-117859) #117912