Change VerifyDiagnosticConsumer so that it *must* contain at least one...

Change VerifyDiagnosticConsumer so that it *must* contain at least one "expected-*" directive.  As a result, for test-cases that are not expected to generate any diagnostics, an additional directive "expected-no-diagnostics" has been implemented which can then be included in such test-cases.  This new directive may not be used in conjunction with any other "expected-*" directive.

This change was initially proposed as a solution to the problem highlighted by check-in r164677, i.e. that -verify will not cause a test-case failure where the compile command does not actually reference the file.

Patch reviewed by David Blaikie.

llvm-svn: 166281