[analyzer] Fix unreachable creating PathDiagnosticLocation with widen-loops=true

In the original design of the analyzer, it was assumed that a BlockEntrance
doesn't create a new binding on the Store, but this assumption isn't true when
'widen-loops' is set to true. Fix this by finding an appropriate location
BlockEntrace program points.

Patch by Henry Wong!

Differential Revision: https://reviews.llvm.org/D37187

llvm-svn: 319333