Revert "Remove unused member (& consequently unused parameter) in SA's Call code."

...and instead add an accessor. We're not using this today, but it's something
that should probably stay in the source for potential clients, and it doesn't
cost a lot. (ObjCPropertyAccess is only created on the stack, and right now
there's only ever one alive at a time.)

This reverts r159581 / commit 8e674e1da34a131faa7d43dc3fcbd6e49120edbe.

llvm-svn: 159595