DebugInfo: Test that the loop backedge in a range-for is attribute to the loop header.

This already works, but somewhat by accident (due to the order of
emission in clang, the location is set to the loop header (during the
emission of the iteratior increment) before the loop backedge is
emitted), so let's just add a test for symmetry and future-proofing.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216298 91177308-0d34-0410-b5e6-96231b3b80d8