[clang] [Clang][C++20] Implement constexpr std::bit_cast for bit-fields (PR #74775)

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sethp wrote:

oh, @tbaederr I see you've done some overlapping work over in https://github.com/llvm/llvm-project/pull/68288 : I'll make time to review that and see what ought to happen to dovetail these patches.

https://github.com/llvm/llvm-project/pull/74775