Fixing a compiler assertion with zero-width bit-fields in packed structs.

According to the GNU docs, zero-sized bitfields should not be affected by the
packed attribute.

Differential Revision: http://llvm-reviews.chandlerc.com/D2693



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201288 91177308-0d34-0410-b5e6-96231b3b80d8