Speed up find-unused-diagnostics. Now runs in less than a second instead of more than a minute.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172330 91177308-0d34-0410-b5e6-96231b3b80d8
Prune an unused diagnostic, detected after improving the script in r172330


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172331 91177308-0d34-0410-b5e6-96231b3b80d8