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/tests/j+k/collectionOverrides/irrelevantCharAtAbstract.kt

@@ -0,0 +1,19 @@
+// FILE: A.java
+
+public abstract class A {
+    abstract public char charAt(int i);
+}
+
+// FILE: B.java
+
+abstract public class B extends A implements CharSequence {
+    public char charAt(int i) { return '1'; }
+}
+
+// FILE: main.kt
+
+abstract class C1 : B()
+
+abstract class C2 : B() {
+    override fun get(index: Int) = '1'
+}