[RFE] Provide warning for C++ class without anchor method

[llvmbot]

Bugzilla Link	21119
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

LLVM's style guide requires classes with vtables to have an out-of-line anchor method (even it requires defining an unused synthetic one).

It would be nice if Clang had a warning to identify C++ classes without a vtable anchor. To reduce noise, it might make sense to limit this to classes with at least one out-of-line method, and then only fire the diagnostic when an implementation of that method is seen.