No override check in FirClassUseSiteScope, fix nasty substitution bug

This fixes MPP override test
(see mppFakeOverride in FirMultiModuleResolveTestGenerated)

idea/testData/fir/multiModule/mppFakeOverrides/common/common.kt

@@ -3,8 +3,9 @@ expect open class A<T>() {
 }
 open class B : A<String>() {
     // Fake: override fun foo(arg: String) = super.foo(arg)
-    // Fake (JVM only) (?): override fun bar(arg: String): String = super.bar(arg)
+    // Fake (JVM only): override fun bar(arg: String): String = super.bar(arg)
 }
 open class C : B() {
-    open fun bar(arg: CharSequence): String = arg.toString()
+    open fun bar(arg: String): String = arg
+    open fun baz(arg: CharSequence): String = arg.toString()
 }

idea/testData/fir/multiModule/mppFakeOverrides/common/common.txt

@@ -12,8 +12,12 @@ FILE: common.kt
     public open class C : R|B| {
         public constructor(): super<R|B|>()
 
-        public open function bar(arg: R|kotlin/CharSequence|): R|kotlin/String| {
-            return@@@bar <Unresolved name: arg>#.<Unresolved name: toString>#()
+        public open function bar(arg: R|kotlin/String|): R|kotlin/String| {
+            return@@@bar <Unresolved name: arg>#
+        }
+
+        public open function baz(arg: R|kotlin/CharSequence|): R|kotlin/String| {
+            return@@@baz <Unresolved name: arg>#.<Unresolved name: toString>#()
         }
 
     }