[clang] [clang] Implement constexpr bit_cast for vectors (PR #66894)

[Richard Smith via cfe-commits]

https://github.com/zygoloid commented:

Thanks, I like the code reuse! There are some minor optimization and simplification opportunities in the code you're reusing that might be worth addressing now.

I'm worried that vectors with padding (which I think is only vectors of x86 `long double`) aren't being treated properly by `bit_cast` here. It's probably a bit awkward to fix, given the new approach, but maybe we can handle the padding separately from the conversion of the vector to/from `APInt`.

https://github.com/llvm/llvm-project/pull/66894