This reverts commit 54db16221c92eb52efbea90ad5b5d2a1d00cda3e.
Check for existence of __SIZOEF_INT128__ so we don't run those
tests on targets that don't have int128.
This is a subset of #68288, with hopefully narrower scope. It does not
support bitcasting to non-integral types yet.
Bitfields are supported, but only if they result in a full byte-sized
final buffer. It does not support casting from null-pointers yet or
casting from indeterminate bits.
The tests are from #68288 and partially from #74775.
The `BitcastBuffer` struct is currently always working in single bits,
but I plan to (try to) optimize this for the common full-byte case.
