MS ABI: Bitfields FielDecls only align if they allocate

Don't consider a __declspec(align) on a bitfield's declaration if it didn't
allocate any underlying storage.

This fixes PR19414.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@206132 91177308-0d34-0410-b5e6-96231b3b80d8