[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

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/references/FirReferenceResolveHelper.kt

@@ -77,7 +77,7 @@ internal object FirReferenceResolveHelper {
                 val callee = calleeReference.resolvedSymbol.fir as? FirCallableDeclaration
                 // TODO: check callee owner directly?
                 if (callee !is FirConstructor && callee?.isStatic != true) {
-                    classLikeDeclaration.companionObject?.let { return it.buildSymbol(symbolBuilder) }
+                    classLikeDeclaration.companionObjectSymbol?.let { return it.fir.buildSymbol(symbolBuilder) }
                 }
             }
         }
@@ -398,7 +398,7 @@ internal object FirReferenceResolveHelper {
         symbolBuilder: KtSymbolByFirBuilder
     ): Collection<KtSymbol> {
         val referencedSymbol = if (fir.resolvedToCompanionObject) {
-            (fir.symbol?.fir as? FirRegularClass)?.companionObject?.symbol
+            (fir.symbol?.fir as? FirRegularClass)?.companionObjectSymbol
         } else {
             fir.symbol
         }

analysis/analysis-api-fir/src/org/jetbrains/kotlin/analysis/api/fir/symbols/KtFirNamedClassOrObjectSymbol.kt

@@ -73,7 +73,7 @@ internal class KtFirNamedClassOrObjectSymbol(
     override val isExternal: Boolean get() = firRef.withFir { it.isExternal }
 
     override val companionObject: KtFirNamedClassOrObjectSymbol? by firRef.withFirAndCache { fir ->
-        fir.companionObject?.let { builder.classifierBuilder.buildNamedClassOrObjectSymbol(it) }
+        fir.companionObjectSymbol?.let { builder.classifierBuilder.buildNamedClassOrObjectSymbol(it.fir) }
     }
 
     override val superTypes: List<KtType> by cached {