[clang] [clang] Better bitfield access units (PR #65742)

[Nathan Sidwell via cfe-commits]

urnathan wrote:

@rjmccall thanks, here is an update with those changes.  I've collapsed it to the 3 commits mentioned earlier

1) Tests marked up for the current behaviour
2) TargetInfo strict/flexible alignment load predicate
3) The new algorithm

https://github.com/llvm/llvm-project/pull/65742