Don't use module internal implementation details in our decorators.

We tried implementing something akin to a conditionalExpectedFailure
decorator for unittest2.  We did this by making use of some
implementation details of the unittest2 module.  In an effort to make
this work with unittest, this patch removes the reliance on the
implementation details.  I have a hard time wrapping my head around
how this all works with the deeply nested decorators, but the spirit
of the patch here is to do do the following: If the condition function
is true, use the original unittest2.expectedFailure decorator.  Otherwise
don't use any decorator, just call the test function.

Differential Revision: http://reviews.llvm.org/D14406
Reviewed By: tberghammer, labath

llvm-svn: 252326