Re-apply r174919 - smarter copy/move assignment/construction, with fixes for
bitfield related issues.

The original commit broke Takumi's builder. The bug was caused by bitfield sizes
being determined by their underlying type, rather than the field info. A similar
issue with bitfield alignments showed up on closer testing. Both have been fixed
in this patch.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175389 91177308-0d34-0410-b5e6-96231b3b80d8