Weaken an assertion in memcpyization to account for
unnamed bitfields.

Unnamed bitfields won't have an explicit copy operation
in the AST, which breaks the strong form of the invariant.

rdar://13816940

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@181289 91177308-0d34-0410-b5e6-96231b3b80d8