Check for type parameters on original type when getting type argument completions (microsoft#46614)

* Add failing test.

* Check for type parameters on the original type when asking for the type argument constraint.

* Update tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts

Author: DanielRosenwasser

src/compiler/checker.ts

@@ -34729,7 +34729,8 @@ namespace ts {
         function getTypeArgumentConstraint(node: TypeNode): Type | undefined {
             const typeReferenceNode = tryCast(node.parent, isTypeReferenceType);
             if (!typeReferenceNode) return undefined;
-            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode)!; // TODO: GH#18217
+            const typeParameters = getTypeParametersForTypeReference(typeReferenceNode);
+            if (!typeParameters) return undefined;
             const constraint = getConstraintOfTypeParameter(typeParameters[typeReferenceNode.typeArguments!.indexOf(node)]);
             return constraint && instantiateType(constraint, createTypeMapper(typeParameters, getEffectiveTypeArguments(typeReferenceNode, typeParameters)));
         }

tests/cases/fourslash/stringLiteralTypeCompletionsInTypeArgForNonGeneric1.ts

@@ -0,0 +1,10 @@
+/// <reference path="./fourslash.ts" />
+
+////interface Foo {}
+////type Bar = {};
+////
+////let x: Foo<"/*1*/">;
+////let y: Bar<"/*2*/">;
+
+verify.completions({ marker: test.markers() });
+