OpenMP Initial testsuite change to purely llvm-lit based testing

This change introduces a check-libomp target which is based upon llvm's lit
test infrastructure. Each test (generated from the University of Houston's
OpenMP testsuite) is compiled and then run. For each test, an exit status of 0
indicates success and non-zero indicates failure. This way, FileCheck is not
needed. I've added a bit of logic to generate symlinks (libiomp5 and libgomp)
in the build tree so that gcc can be tested as well.  When building out-of-
tree builds, the user will have to provide llvm-lit either by specifying
-DLIBOMP_LLVM_LIT_EXECUTABLE or having llvm-lit in their PATH.

Differential Revision: http://reviews.llvm.org/D11821

llvm-svn: 248211