[analyzer] Handle implicit statements used for end-of-path nodes' source locs.

Some implicit statements, such as the implicit 'self' inserted for "free"
Objective-C ivar access, have invalid source locations. If one of these
statements is the location where an issue is reported, we'll now look at
the enclosing statements for a valid source location.

<rdar://problem/12446776>

llvm-svn: 165354