DebugInfo: Remove some now-unnecessary location handling around function arguments.

r225000 generalized debug info line info handling for expressions such
that this code is no longer necessary.

This removes the last use of CGDebugInfo::getLocation, but not all the
uses of CGDebugInfo::CurLoc, which is still used internally in
CGDebugInfo. I'd like to do away with all of that & might succeed after
a few more patches.

llvm-svn: 225085