refactor(ngcc): share code between CommonJsReflectionHost and UmdReflectionHost (#34512)

While different, CommonJS and UMD have a lot in common regarding the
their exports are constructed. Therefore, there was some code
duplication between `CommonJsReflectionHost` and `UmdReflectionHost`.

This commit extracts some of the common bits into a separate file as
helpers to allow reusing the code in both `ReflectionHost`s.

PR Close #34512

packages/compiler-cli/ngcc/src/utils.ts

@@ -66,6 +66,13 @@ export function findAll<T>(node: ts.Node, test: (node: ts.Node) => node is ts.No
   }
 }
 
+export function getOrDefault<K, V>(map: Map<K, V>, key: K, factory: (key: K) => V): V {
+  if (!map.has(key)) {
+    map.set(key, factory(key));
+  }
+  return map.get(key) !;
+}
+
 /**
  * Does the given declaration have a name which is an identifier?
  * @param declaration The declaration to test.