Add additional dependency to clang/clang-headers/FileCheck to avoid possible troubles with in-tree build/test of libomp + allow parallel testing of libomp. Also includes bugfixes for tests + improvements to avoid possible race conditions. Differential Revision: http://reviews.llvm.org/D14055 git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@251797 91177308-0d34-0410-b5e6-96231b3b80d8