[CMake] Re-enable libomptarget and restrict tests to Clang 6.0.0

We have just fixed the codegen of omp_is_initial_device() to reliably work
when offloading to the same device, see commit r316001. This fixes the
failing tests that were the reason why we disabled the library for 5.0.

Differential Revision: https://reviews.llvm.org/D39052

llvm-svn: 318847