Introduce FunctionDescriptor.initialSignatureDescriptor

Mostly it's needed for backend to extract descriptor with initial signature

For example:
class A implements CharSequence {
        char charAt(int index) {}
}

We see `charAt` this method as `fun get(index: Int): Char`, but in backend it
matters what signature had this descriptor before.

core/descriptor.loader.java/src/org/jetbrains/kotlin/load/java/descriptors/JavaMethodDescriptor.java

@@ -147,7 +147,7 @@ public class JavaMethodDescriptor extends SimpleFunctionDescriptorImpl implement
                 isOperator(), isInfix(), isExternal(), isInline(), isTailrec(), getOriginal(),
                 /* copyOverrides = */ true, getKind(),
                 enhancedValueParameters, enhancedReceiverType, enhancedReturnType,
-                null, false);
+                null, /* preserveSource */false, /* signatureChange = */ false);
 
         assert enhancedMethod != null : "null after substitution while enhancing " + toString();
         return enhancedMethod;

core/descriptor.loader.java/src/org/jetbrains/kotlin/load/java/descriptors/JavaPropertyDescriptor.java

@@ -77,6 +77,7 @@ public class JavaPropertyDescriptor extends PropertyDescriptorImpl implements Ja
             newGetter = new PropertyGetterDescriptorImpl(
                     enhanced, getter.getAnnotations(), getter.getModality(), getter.getVisibility(),
                     getter.hasBody(), getter.isDefault(), getter.isExternal(), getKind(), getter, getter.getSource());
+            newGetter.setInitialSignatureDescriptor(getter.getInitialSignatureDescriptor());
             newGetter.initialize(enhancedReturnType);
         }
 
@@ -86,6 +87,7 @@ public class JavaPropertyDescriptor extends PropertyDescriptorImpl implements Ja
             newSetter = new PropertySetterDescriptorImpl(
                     enhanced, setter.getAnnotations(), setter.getModality(), setter.getVisibility(),
                     setter.hasBody(), setter.isDefault(), setter.isExternal(), getKind(), setter, setter.getSource());
+            newSetter.setInitialSignatureDescriptor(newSetter.getInitialSignatureDescriptor());
             newSetter.initialize(setter.getValueParameters().get(0));
         }
 

core/descriptor.loader.java/src/org/jetbrains/kotlin/load/java/lazy/descriptors/LazyJavaClassMemberScope.kt

@@ -396,12 +396,16 @@ public class LazyJavaClassMemberScope(
                 propertyDescriptor, getterMethod.annotations, /* isDefault = */false,
                 /* isExternal = */ false, getterMethod.source
         ).apply {
+            initialSignatureDescriptor = getterMethod
             initialize(propertyDescriptor.type)
         }
 
         val setter = setterMethod?.let { setterMethod ->
             DescriptorFactory.createSetter(propertyDescriptor, setterMethod.annotations, /* isDefault = */false,
-            /* isExternal = */ false, setterMethod.visibility, setterMethod.source)
+            /* isExternal = */ false, setterMethod.visibility, setterMethod.source
+            ).apply {
+                initialSignatureDescriptor = setterMethod
+            }
         }
 
         return propertyDescriptor.apply { initialize(getter, setter) }