Devirtualize destructor of final class.

Summary:
Take advantage of the final keyword to devirtualize destructor calls.

Fix https://bugs.llvm.org/show_bug.cgi?id=21368

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: davidxl, Prazek, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63161

llvm-svn: 364100