[libc++] Divorce the std Lit feature from the -std=XXX compiler flag

After this patch, we can use `--param std=c++20` even if the compiler only
supports -std=c++2a. The test suite will handle that for us. The only Lit
feature that isn't fully baked will always be the "in development" one,
since we don't know exactly what year the standard will be ratified in.

This is another take on https://reviews.llvm.org/D99789.

Differential Revision: https://reviews.llvm.org/D100210

GitOrigin-RevId: 8508b1c133f6dea558fd62f73cf333bdec809026
14 files changed