PR26547: alignof should return ABI alignment, not preferred alignment

PR26547: alignof should return ABI alignment, not preferred alignment

Summary:
- Add `UETT_PreferredAlignOf` to account for the difference between `__alignof` and `alignof`
- `AlignOfType` now returns ABI alignment instead of preferred alignment iff clang-abi-compat > 7, and one uses _Alignof or alignof

Patch by Nicole Mazzuca!

Differential Revision: https://reviews.llvm.org/D53207

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345419 91177308-0d34-0410-b5e6-96231b3b80d8