[analyzer] Add experimental option "leak-diagnostics-reference-allocation".

This is an opt-in tweak for leak diagnostics to reference the allocation
site if the diagnostic consumer only wants a pithy amount of information,
and not the entire path.

This is a strawman enhancement that I expect to see some experimentation
with over the next week, and can go away if we don't want it.

Currently it is only used by RetainCountChecker, but could be used
by MallocChecker if and when we decide this should stay in.