Use CMake's cmake_parse_arguments() instead.
It's called in a slightly different way, but supports all our use cases.
It's in CMake 2.8.8, which is our minimum supported version.
CMake 3.0 doc (roughly the same. No direct link to 2.8.8 doc):
Finally, delete LLVM's parse_arguments() definition.
This is dependent on http://reviews.llvm.org/D10529