Update VerifyDiagnosticConsumer to only get directives during parsing.

The old behavior was to re-scan any files (like modules) where we may have
directives but won't actually be parsing during the -verify invocation.
Now, we keep the old behavior in Debug builds as a sanity check (though
modules are a known entity), and expect all legitimate directives to come
from comments seen by the preprocessor.

This also affects the ARC migration tool, which captures diagnostics in
order to filter some out. This change adds an explicit cleanup to
CaptureDiagnosticsConsumer in order to let its sub-consumer handle the
real end of diagnostics.

This was originally split into four patches, but the tests do not run
cleanly without all four, so I've combined them into one commit.

Patches by Andy Gibbs, with slight modifications from me.

llvm-svn: 161650