[VerifyDiagnosticConsumer] support -verify=<prefixes>

This mimics FileCheck's --check-prefixes option.

The default prefix is "expected". That is, "-verify" is equivalent to
"-verify=expected".

The goal is to permit exercising a single test suite source file with different
compiler options producing different sets of diagnostics.  While cpp can be
combined with the existing -verify to accomplish the same goal, source is often
easier to maintain when it's not cluttered with preprocessor directives or
duplicate passages of code. For example, this patch also rewrites some existing
clang tests to demonstrate the benefit of this feature.

Patch by Joel E. Denny, thanks!

Differential Revision: https://reviews.llvm.org/D39694

llvm-svn: 320908