Somehow I dropped this in r233585, and we haven't had `DEBUG_LOC_AGAIN`
records since. Add it back. Also tests that the output assembly looks
okay.
Fixes PR23436.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236661 91177308-0d34-0410-b5e6-96231b3b80d8