Recognize simple cases of trivial explicitly defaulted public dtors as irrelevant

Reviewed in http://llvm-reviews.chandlerc.com/D3190

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204953 91177308-0d34-0410-b5e6-96231b3b80d8
Don't emit exit-time destructor warnings for trivial explicitly defaulted dtors

This commit also adds an additional test case for the global destructor warning.

Reviewed in http://llvm-reviews.chandlerc.com/D3205

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204954 91177308-0d34-0410-b5e6-96231b3b80d8