Skipped IncompleteModulesTestCase on OS X

This is hitting an assert in clang when evaluating the
module load.  I am seeing it locally on Xcode 7.3 public Beta 1
and on the llvm.org Green Dragon buildbot supposedly running
Xcode 7.0.

Tracked by:
https://llvm.org/bugs/show_bug.cgi?id=26267

llvm-svn: 258602