[libc++] Add availability markup for LLVM 19 and LLVM 20 (#140072)

An LLVM 19-aligned libc++ was released with macOS 15.4 (and corresponding OSes),
and LLVM-20 aligned with macOS 26.0. This patch adds availability markup to
reflect that.

NOKEYCHECK=True
GitOrigin-RevId: 3252e11da3ac990732d02d1a881b6544d81fe955
2 files changed