A couple of cleanup skipifs

I added a line to constOverflow.skipif to catch the case when the target compiler is "clang" (on darwin).
I added the execopts/bradc/gdbddash/declint2.skipif to skip that test when --fast is tossed on the Chapel command line.
There's already a suppression for this test in test/Suppressions/fast.suppress, is there a reason to skip it altogether?