CFGBuilder: Fix crash when visiting a range-based for over a dependent type

Summary:
CFG generation is expected to fail in this case, but it should not crash.

Also added a test that reproduces the crash.

Reviewers: klimek

Subscribers: cfe-commits

Patch by Martin Boehme!

Differential Revision: http://reviews.llvm.org/D21895

llvm-svn: 274834