Add a virtual destructor to a class with virtual methods.

Summary:
Recently, Clang enabled the check for virtual destructors
in the presence of virtual methods. That broke the bootstrap
build. Fixing it.

Reviewers: pcc

Reviewed By: pcc

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D31776

llvm-svn: 299672