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

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huixie90 wrote:

@efriedma-quic  could you please have a look at the updated version? it works for bit field.  May I have some help on how to write these IR tests?  

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