FIR IDE: quickfix to add override keyword

Also enables more tests that covers previous quickfixes

compiler/testData/diagnostics/tests/subtyping/extensionOnLocalClasses.fir.kt

@@ -0,0 +1,9 @@
+package foo.bar
+
+fun test() {
+    class A {
+        inner class B
+    }
+
+    fun <!UNRESOLVED_REFERENCE!>A.B<!>.foo() {}
+}