CMake: Stop using LLVM's custom parse_arguments (delete implementation). NFC

Summary:
Finally, delete LLVM's parse_arguments() definition.
Second part of D10531.

This is dependent on http://reviews.llvm.org/D10529

Reviewers: pcc, beanz, chapuni

Subscribers: llvm-commits

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

llvm-svn: 240122