[OPENMP] Add dependency to clang/clang-headers etc. for in-tree build of libomp.

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

llvm-svn: 251797