Correct OptionCategoryCompare() in the command line library.

Summary:
It should return <0, 0, or >0 for less-than, equal, and greater-than like
strcmp() (according to the history, it used to be implemented with
strcmp()) but it actually returned 0, or 1 for not-equal and equal.

Reviewers: qcolombet

Reviewed By: qcolombet

Subscribers: qcolombet, llvm-commits

Differential Revision: https://reviews.llvm.org/D30996

llvm-svn: 298844