Emit invariant.group.barrier when using union field

Summary:
We need to emit barrier if the union field
is CXXRecordDecl because it might have vptrs. The testcode
was wrongly devirtualized. It also proves that having different
groups for different dynamic types is not sufficient.

Reviewers: rjmccall, rsmith, mehdi_amini

Subscribers: amharc, cfe-commits

Differential Revision: https://reviews.llvm.org/D31830

llvm-svn: 304448