swift234

two unrelated changes:
 - Have DiagnosticEngine produce "aka" annotations for sugared types.
 - Fix the "optional type '@lvalue C?' cannot be used as a boolean; test for '!= nil' instead"
   diagnostic to stop printing @lvalue noise.

This addresses:
<rdar://problem/19036351> QoI: Print minimally-desugared 'aka' types like Clang does



Swift SVN r30587