[clang] [libc++] Add builtin to clear padding bytes (prework for P0528R3) (PR #75371)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Also, please fix the title so it isn't prefixed with "[libc++]"; the intent is for this to be used in libc++, but it's proposing a clang intrinsic.

https://github.com/llvm/llvm-project/pull/75371