Fix an invalid check for duplicate option categories.

Summary:
The check performed in the comparator is invalid, as some STL
implementations enforce strict weak ordering by calling the comparator with the
same value. This check was also in a wrong place: the assertion would only fire
when -help was used. The new check is performed each time the category is
registered (we are not going to have thousands of them, so it's fine to do it in
O(N^2)).

Reviewers: jordan_rose

Reviewed By: jordan_rose

CC: cfe-commits, alexmc

Differential Revision: http://llvm-reviews.chandlerc.com/D2699