Get ARCMT/GC-check-warn-nsalloc.m working

The -no-ns-alloc-error migration option now causes the diagnostic to be ignored
completely. If this isn't desired, the error can be downgraded to a warning
using the usual -Wno-error=arcmt-ns-alloc.

Note that we can't use -verify right now on this test because
VerifyDiagnosticConsumer gets confused by multiple SourceManager instances,
which is presumably the reason it was XFAILed in the first place and why the
regression wasn't detected. We'll grep instead for now.