Make detect_invalid_pointer_pairs option to be tristate.

Summary:
With the change, one can choose not to report comparison (or subtraction)
of a pointer with nullptr pointer.

Reviewers: kcc, jakubjelinek, alekseyshl

Reviewed By: alekseyshl

Subscribers: kubamracek

Differential Revision: https://reviews.llvm.org/D41479

llvm-svn: 323995