[clang-tidy] cppcoreguidelines-virtual-base-class-destructor: a new check

Finds base classes and structs whose destructor is neither public and
virtual nor protected and non-virtual.
A base class's destructor should be specified in one of these ways to
prevent undefined behaviour.

Fixes are available for user-declared and implicit destructors that are
either public and non-virtual or protected and virtual.

This check implements C.35 [1] from the CppCoreGuidelines.

Reviewed By: aaron.ballman, njames93

Differential Revision: http://reviews.llvm.org/D102325

  [1]: http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Rc-dtor-virtual