[analyzer][NFC] Remove duplicated work from retain count leak report

Allocation site is the key location for the leak checker.  It is a
uniqueing location for the report and a source of information for
the warning's message.

Before this patch, we calculated and used it twice in bug report and
in bug report visitor.  Such duplication is not only harmful
performance-wise (not much, but still), but also design-wise.  Because
changing something about the end piece of the report should've been
repeated for description as well.

Differential Revision: https://reviews.llvm.org/D100626