DebugInfo: More accurate line information for placement new.

This actually came up as a break in UBSan tests (look for a follow-up
commit to this one to see the UBSan test fallout) when I tried a broader
fix to location information.

I have some other ideas about how to do that broader change & will keep
looking into it.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@224221 91177308-0d34-0410-b5e6-96231b3b80d8