[libc++][CI] Updates to Clang 19. (#85301)

Since we have released Clang 16 is no longer actively supported. However
the FreeBSD runner is still using this, so some tests still guard
against Clang 16.

GitOrigin-RevId: b798c2af8b8888f8db2a9ec293066fb72d893f0c
5 files changed