[analyzer] When reporting a leak in RetainCount checker due to an early exit...

[analyzer] When reporting a leak in RetainCount checker due to an early exit from init, step into init.

The heuristic here (proposed by Jordan) is that, usually, if a leak is due to an early exit from init, the allocation site will be
a call to alloc. Note that in other cases init resets self to [super init], which becomes the allocation site of the object.

llvm-svn: 179221