[clang] [clang][bytecode] Start implementing __builtin_bit_cast (PR #112126)

[Timm Baeder via cfe-commits]

tbaederr wrote:

Victory!

 1) Iterate composite fields backwards for big-endian targets
 2) Fix `BitcatBuffer::data()` pointing to the wrong byte if the number of bytes in the buffer is not a multiple of `sizeof(uint64_t)` (which is what `llvm::BitBuffer` uses internally) and we're on a big-endian host.
 3) Validate that all of this works on little-endian and big-endian hosts, with asan and ubsan enabled.



https://github.com/llvm/llvm-project/pull/112126