For now, *_STANDALONE_BUILD is set to ON even if they're built along
with LLVM because of issues mentioned in the comments. This can cause some issues.
For example, if we build OpenMP along with LLVM, we'd like to copy those OpenMP
headers to <prefix>/lib/clang/<version>/include such that clang can find
those headers without using -I <prefix>/include because those headers will be
copied to <prefix>/include if it is built standalone.
In this patch, we fixed the dependence issue in OpenMP such that it can be built
correctly even with OPENMP_STANDALONE_BUILD=OFF. The issue is in the call to
add_lit_testsuite, where clang and clang-resource-headers are passed as
DEPENDS. Since we're building OpenMP along with LLVM, clang is set by CMake
to be the C/C++ compiler, therefore these two dependences are no longer needed,
where caused the dependence issue.