Suppress constOverflow test when CHPL_TARGET_COMPILER==clang.
Add declint2.skipif to skip when --fast is tossed.

This test (declint2) is intende to show that GDB will stop when a deliberate
out-of-bounds array access is attempted.  But bounds checking is turned off when
--fast is tossed, so this test does not pertain.
Merge pull request #725 from hildeth/master

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.