[CSClosure] Declarations prefixed with $ can act as inferrable placeholders

If the type for such declaration could be completely inferred it
would be used during solution application. This is used to infer
types for result builder components.

lib/Sema/ConstraintSystem.cpp

@@ -894,6 +894,18 @@ Type ConstraintSystem::replaceInferableTypesWithTypeVars(
               TVO_CanBindToNoEscape | TVO_PrefersSubtypeBinding |
                   TVO_CanBindToHole);
         }
+
+        if (auto *var = placeholderTy->getOriginator().dyn_cast<VarDecl *>()) {
+          if (var->getName().hasDollarPrefix()) {
+            auto *repr =
+                new (type->getASTContext()) PlaceholderTypeRepr(var->getLoc());
+            return createTypeVariable(
+                getConstraintLocator(locator,
+                                     LocatorPathElt::PlaceholderType(repr)),
+                TVO_CanBindToNoEscape | TVO_PrefersSubtypeBinding |
+                    TVO_CanBindToHole);
+          }
+        }
       }
 
       return type;