[Sema] Commit a better fix for r240242

[Sema] Commit a better fix for r240242

Skip calls to HasTrivialDestructorBody() in the case where the
destructor is never invoked. Alternatively, Richard proposed to change
Sema to declare a trivial destructor for anonymous union member, which
seems too wasteful.

Differential Revision:	http://reviews.llvm.org/D10508

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240742 91177308-0d34-0410-b5e6-96231b3b80d8