DebugInfo: Simplify/correct test for correct constant emission when dealing with derived types.

This test was using the inliner and other optimizations to test a case
that's actually a bug anyway. Bug and possible fix/discussion described
here ( http://reviews.llvm.org/D3714 ).

But the functionality that was implemented along with this test is still
desired, so simplify the test to verify a more obvious/less wrong case
that the functionality addressed: looking through const sugar to the
underlying type when emitting a constant (so the constant is emitted as
signed/unsigned as appropriate depending on the signedness of the
underlying type).