fix(language-service): guard access to Symbol.members (#15529)

Fixes #15528 

What is the current behavior?
The language service access TypeScript's Symbol.members without checking for null or undefined.
What is the new behavior?
The access is guarded.

Author: chuckjaz

packages/language-service/src/typescript_host.ts

@@ -668,7 +668,7 @@ class TypeScriptSymbolQuery implements SymbolQuery {
   getSpanAt(line: number, column: number): Span { return spanAt(this.source, line, column); }
 
   private getTemplateRefContextType(type: ts.Symbol): ts.Symbol {
-    const constructor = type.members['__constructor'];
+    const constructor = type.members && type.members['__constructor'];
     if (constructor) {
       const constructorDeclaration = constructor.declarations[0] as ts.ConstructorTypeNode;
       for (const parameter of constructorDeclaration.parameters) {