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

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huixie90 wrote:

> If you want to modify part of a bitfield unit, you need to load/store the whole bitfield unit, as computed by the CGRecordLayout. This is true whether you're modifying an actual field, or padding adjacent to a field. Since any padding has to be adjacent to a bitfield, you can get the relevant information out of the CGBitFieldInfo for that bitfield.

Hello, this is now working with the bitfield. Iterating over the field of a struct also gives me information about where bit fields are

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