Revert "Revert r178079, it caused PR15637."

This reverts commit r178497 since the backend has been fixed.

Also add a test to ensure that we're emitting template information for unions.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178587 91177308-0d34-0410-b5e6-96231b3b80d8