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

[Louis Dionne via cfe-commits]

ldionne wrote:

@AaronBallman We're struggling a bit to find the right person to help review this -- do you know who would be a good candidate? Hui is very familiar with libc++ but he's less familiar with codegen, and I'm not very useful in that area of the project :-).

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