TableGen: Give up on exact fixits for diagnostic groups

With recent changes in the TableGen frontend, we no longer have usable
location information for anonymous defs.

Fixes test breakage caused by r326788.

The normal, non-error TableGen output is not affected by this change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@326822 91177308-0d34-0410-b5e6-96231b3b80d8