This mimics FileCheck's --check-prefixes option.
The default prefix is "expected". That is, "-verify" is equivalent to
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.