[FIR] Make FirRegularClass.companionObject companionObjectSymbol field

This is needed for two reasons:
1. Before this change companion object appeared in FirRegularClass
  twice: in declarations list and in companionObject field. This may
  trigger twice transform of it
2. It's very hard to implement generation of companion object by plugins
  because if it is part of the tree then generated declaration must be
  registered in FirProvider, which is inconsistent with other generated
  declarations. Replacing FIR with symbol and removing custom logic of
  visiting/transforming companion FIR allows us to just replace companionSymbol
  in FirClass if plugin wants to generate it without any additional work

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/declarations/ImplicitReceiverUtils.kt

@@ -54,7 +54,7 @@ data class ImplicitReceivers(
 fun SessionHolder.collectTowerDataElementsForClass(owner: FirClass, defaultType: ConeKotlinType): TowerElementsForClass {
     val allImplicitCompanionValues = mutableListOf<ImplicitReceiverValue<*>>()
 
-    val companionObject = (owner as? FirRegularClass)?.companionObject
+    val companionObject = (owner as? FirRegularClass)?.companionObjectSymbol?.fir
     val companionReceiver = companionObject?.let { companion ->
         ImplicitDispatchReceiverValue(
             companion.symbol, session, scopeSession
@@ -72,9 +72,9 @@ fun SessionHolder.collectTowerDataElementsForClass(owner: FirClass, defaultType:
             ?.asTowerDataElement(isLocal = false)
             ?.let(superClassesStaticsAndCompanionReceivers::add)
 
-        (superClass as? FirRegularClass)?.companionObject?.let { companion ->
+        (superClass as? FirRegularClass)?.companionObjectSymbol?.let {
             val superCompanionReceiver = ImplicitDispatchReceiverValue(
-                companion.symbol, session, scopeSession
+                it, session, scopeSession
             )
 
             superClassesStaticsAndCompanionReceivers += superCompanionReceiver.asTowerDataElement()