LLVM Clang 19 Adds Initial "-std=c2y" Support For The Next C Standard

LLVM Clang 19 this morning landed initial support for the -std=c2y and -std=gnu2y options for targeting the next C programming language standard.

LLVM Clang 19 is now honoring -std=c2y and -std=gnu2y for targeting the next in-progress C standards. This next revision will likely be called "C26" but as the 2026 publication isn't yet guaranteed, for the time being only the "C2Y" placeholder is being recognized and honored along with GNU2Y as the GNU dialect of C2Y. LLVM Clang 19 also bumps the __STDC_VERSION__ macro to 202400L for reflecting the newer revision.

For now there isn't much to the C2Y support in LLVM/Clang but with time will obviously evolve more past C23 and as more C2Y proposals are published. The C2Y support in LLVM can be tracked via the C status page.

This commit is what adds the initial C2Y/GNU2Y targeting today to LLVM Clang 19 Git.