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

[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>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@165354 91177308-0d34-0410-b5e6-96231b3b80d8