[CSDiagnostics] Handle try? contextual mismatch directly

It detends on a situation whether try? would get a type inferred
so the mismatch is against a contextual type, or contextual type
would be used as a type of `try?` and fail comparsion with inner
expression type. In either case the mismatch is contextual.

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -1878,6 +1878,12 @@ bool ContextualFailure::diagnoseAsError() {
     if (diagnoseCoercionToUnrelatedType())
       return true;
 
+    if (isa<OptionalTryExpr>(anchor)) {
+      emitDiagnostic(anchor->getLoc(), diag::cannot_convert_initializer_value,
+                     getFromType(), getToType());
+      return true;
+    }
+
     return false;
   }