[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

llvm-svn: 240742