[analyzer] Improve handling of noreturn destructors

Summary:
The analyzer incorrectly handled noreturn destructors which were hidden inside
function calls. This happened because NoReturnFunctionChecker only listened for
PostStmt events, which are not executed for destructor calls. I've changed it to
listen to PostCall events, which should catch both cases.

Reviewers: jordan_rose

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1056