Add FileCheck -implicit-check-not option to allow stricter tests without adding too many CHECK-NOTs manually.

Summary:
Add FileCheck -implicit-check-not option which allows specifying a
pattern that should only occur in the input when explicitly matched by a
positive check. This feature allows checking tool diagnostics in a way
clang -verify does it for compiler diagnostics.

The option has been tested on a number of clang-tidy checks, I'll post a link to
the clang-tidy patch to this thread.

Once there's an agreement on the general direction, I can add tests and
documentation.

Reviewers: djasper, bkramer

Reviewed By: bkramer

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D4462