Fix signature clash problems caused by special built-ins

Do not treat members with already changed signature as a reason
to create a hidden copy

See tests for clarification:
- There are `charAt` method in B that has different name in Kotlin - `get`,
  i.e. relevant descriptor has initialSignatureDescriptor != null
- When collecting methods from supertypes, `charAt` from A is also get
  transformed to `get`
- So it has effectively the same signature as B.get (already declared)
- If by an accident B.get had been declared with Kotlin signature
  we would have add A.charAt (after transformation) with special flag:
  HiddenToOvercomeSignatureClash (hides it from resolution)
- But here B.charAt was artificially changed to `get`, so no signature clash
  actually happened

 #KT-13730 Fixed

compiler/testData/diagnostics/testsWithJava8/specialBuiltIns/hashtableInheritance.kt

@@ -0,0 +1,4 @@
+class C1 : java.util.Hashtable<String, Int>()
+class C2 : java.util.Hashtable<String, Int>() {
+    override fun get(key: String) = 123
+}