Create a diagnostic group for...

Create a diagnostic group for warn_call_to_pure_virtual_member_function_from_ctor_dtor, so it can be turned into an error using Werror

Summary: Patch by Arnaud Bienner

Reviewers: davide, rsmith, jkorous

Reviewed By: jkorous

Subscribers: jkorous, sylvestre.ledru, cfe-commits

Differential Revision: https://reviews.llvm.org/D53807

llvm-svn: 347555