Fix static analyzer crash when casting from an incomplete type

Summary:
When doing a reinterpret+dynamic cast from an incomplete type, the analyzer
would crash (bug #16308). This fix makes the dynamic cast evaluator ignore
incomplete types, as they can never be used in a dynamic_cast. Also adding a
regression test.

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1006