This commit fixes a crash that happens when clang is analyzing a
transparent_union attribute on a union which has a field with incomplete type.

rdar://28630028

Differential Revision: https://reviews.llvm.org/D25273


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@283432 91177308-0d34-0410-b5e6-96231b3b80d8