Require the containing type to be complete when we see
__alignof__ of a field.

This problem can only happen in C++11.

Also do some petty optimizations.

rdar://13784901

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181185 91177308-0d34-0410-b5e6-96231b3b80d8