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

llvm-svn: 345419
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 4b72633..d276ccc 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -3864,6 +3864,7 @@
     case UETT_SizeOf:
       Out << 's';
       break;
+    case UETT_PreferredAlignOf:
     case UETT_AlignOf:
       Out << 'a';
       break;