CMake: Stop using LLVM's custom parse_arguments. NFC

Summary:
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):
http://www.cmake.org/cmake/help/v3.0/module/CMakeParseArguments.html?highlight=cmake_parse_arguments

Reviewers: pcc, beanz, chapuni

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10531

llvm-svn: 240121