I'm not actually sure how to properly add arguments for clang. I am pretty sure the problem is down to incorrect syntax. For example, when adding "-fno-vectorize" (as seen here) in Properties > Command Line > Additional Options, the compiler tells me
1>clang-cl : warning : unknown argument ignored in clang-cl: '-fno-vectorize' [-Wunknown-argument]
So, the compiler sees the arguments, just doesn't parse them correctly.
For MSVC flags look like "/Qpar" (for example this one turns on auto-parallelization) and it works just fine.