Add regression test

tests/cases/compiler/mutuallyRecursiveCallbacks.ts

@@ -0,0 +1,7 @@
+// Repro from #18277
+
+interface Foo<T> { (bar: Bar<T>): void };
+type Bar<T> = (foo: Foo<T>) => Foo<T>;
+declare function foo<T>(bar: Bar<T>): void;
+declare var bar: Bar<{}>;
+bar = foo;