[OpenMP] Fixed the issue that CMake variables for OpenMP were not passed...

[OpenMP] Fixed the issue that CMake variables for OpenMP were not passed through when building OpenMP with LLVM_ENABLE_RUNTIMES

Currently when building OpenMP along with LLVM, CMake variables for OpenMP (prefix with `LIBOMP` and `LIBOMPTARGET`) will not be passed through because by default it uses the prefix of the runtime name, aka `OPENMP` in this case. This patch fixed this issue.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D93603