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

[via cfe-commits]

DaMatrix wrote:

While attempting to make the result properly endianness-dependent, I noticed that `VectorExprEvaluator::VisitCastExpr` with `CK_BitCast` already had code for this which takes the byte ordering into account (although it also didn't handle bool vectors correctly either). I've now moved the existing Vector<->APInt bit casting code into a pair of separate methods, and made both `CK_BitCast` and the new bit casting code use those.

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