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

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huixie90 wrote:

@efriedma-quic 

I refactored the code which does the following
1. Breadth first search the AST type, fill in "occupied bits intervals"
2. sort and merge the occupied bits interval and work out the "padding" bits intervals
3. clear the padding intervals.  current converting to the byte level and write zeros byte. todo: need to deal with bits that don't occupy the full byte. need to generate instructions like `Byte &= ~PaddingBitMask` . How to generate such an IR instruction ?

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