[Sema] Fix PR30520: Handle incomplete field types in transparent_union unions

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
(cherry picked from commit d9e3029d1906f2991bd7a17282002ee6f3144e16)
2 files changed