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

[Victor Campos via cfe-commits]

vhscampos wrote:

Hi, just FYI that the padding clearing logic falls apart in big endian mode, specifically for _BitInt and bitfields.

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