[OpenMP] Fix library path missing when using OpenMP

The changes in D122444 caused OpenMP programs built with the
LLVM_ENABLE_RUNTIMES options to stop finding the libraries. We generally
expect to link against the libraries associated with the clang
installation itself but we no longer implicitly included that directory.
This patch adds in the include path of the clang installations library
to ensure we can find them.

Reviewed By: jdoerfert, MaskRay

Differential Revision: https://reviews.llvm.org/D122592