Link error occurred when time profiling in libomp is enabled by default
because libomp is assumed to be a C library but the dependence on
libLLVMSupport for profiling is a C++ library. Currently the issue blocks all
OpenMP tests in Phabricator.
This patch set a new CMake option OPENMP_ENABLE_LIBOMP_PROFILING to
enable/disable the feature. By default it is disabled. Note that once time
profiling is enabled for libomp, it becomes a C++ library.