Infer the return type of a closure to be () if it contains no

return statements, or a return statement with no operand.

Also, fix a special-case diagnostic about converting a return
expression to (1) only apply to converting the actual return
expression, not an arbitrary sub-expression, and (2) use the
actual operand and return types, not the drilled-down types
that caused the failure.

Swift SVN r30420

include/swift/AST/AnyFunctionRef.h

@@ -41,6 +41,16 @@ public:
     assert(ACE && "should have a closure");
   }
 
+  /// Construct an AnyFunctionRef from a decl context that's known to
+  /// be some sort of function.
+  static AnyFunctionRef fromFunctionDeclContext(DeclContext *dc) {
+    if (auto fn = dyn_cast<AbstractFunctionDecl>(dc)) {
+      return fn;
+    } else {
+      return cast<AbstractClosureExpr>(dc);
+    }
+  }
+
   CaptureInfo &getCaptureInfo() const {
     if (auto *AFD = TheFunction.dyn_cast<AbstractFunctionDecl *>())
       return AFD->getCaptureInfo();