Make add_new_check.py's insertion of registerCheck<> match the sort order

Summary:
Following on from review comments in D65919 about the ordering
of the registerCheck<> calls. Sort based on the check name which might
be on the line after the registerCheck<>

Reviewers: aaron.ballman

Subscribers: cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66505

llvm-svn: 370527