Fix @skipUnlessPlatform and @skipIfPlatform decorators to work on classes.

The decorators to skip if or unless certain platforms relied on calling
skipTest, which only worked if they were decorating individual test cases.
However for decorating entire classes it needs different behavior. This behavior
is already encapsulated in unittest2.skipIf and unittest2.skipUnless so this
modifies the platform decorators to use these unittest skip decorators.

Test Plan:
./dotest.py -t -p TestObjCMethods2.py
See skipped tests on linux, and see tests run on macosx.

Differential Revision: http://reviews.llvm.org/D8903

llvm-svn: 234514