llvm-project

3252e11d - [libc++] Add availability markup for LLVM 19 and LLVM 20 (#140072)

[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.