[CommandLine] Don't allow duplicate categories.

Summary:
This is a fix to D61574, r360179, that allowed duplicate
OptionCategory's.  This change adds a check to make sure a category can
only be added once even if the user passes it twice.

Reviewed By: MaskRay

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61972

llvm-svn: 360913