Fix brace init of unions with unnamed struct members

The check for unnamed members was intended to skip unnamed bitfields,
but it ended up skipping unnamed structs. This lead to an assertion in
IRGen.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221818 91177308-0d34-0410-b5e6-96231b3b80d8