[analyzer] Silently drop all reports within synthesized bodies.

Much of our diagnostic machinery is set up to assume that the report
end path location is valid. Moreover, the user may be quite confused
when something goes wrong in our BodyFarm-synthesized function bodies,
which may be simplified or modified from the real implementations.
Rather than try to make this all work somehow, just drop the report so
that we don't try to go on with an invalid source location.

Note that we still handle reports whose /paths/ go through invalid
locations, just not those that are reported in one.

We do have to be careful not to lose warnings because of this.
The impetus for this change was an autorelease being processed within
the synthesized body, and there may be other possible issues that are
worth reporting in some way. We'll take these as they come, however.

<rdar://problem/14611722>
[analyzer] Don't process autorelease counts in synthesized function bodies.

We process autorelease counts when we exit functions, but if there's an
issue in a synthesized body the report will get dropped. Just skip the
processing for now and let it get handled when the caller gets around to
processing autoreleases.

(This is still suboptimal: objects autoreleased in the caller context
should never be warned about when exiting a callee context, synthesized
or not.)

Second half of <rdar://problem/14611722>