[FIR2IR] Cache generated IR functions for data/value classes in declarations storage

Previously, DataClassMembersGenerator saved generated functions only in
  symbol table, which led to the situation when there is a FIR function
  in scopes, IR function in symbol table but no record about their relation
  in Fir2IrDeclarationStorage

^KT-61637

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrDeclarationStorage.kt

@@ -352,6 +352,10 @@ class Fir2IrDeclarationStorage(
         delegatedReverseCache[irFunction] = function
     }
 
+    internal fun cacheGeneratedFunction(firFunction: FirSimpleFunction, irFunction: IrSimpleFunction) {
+        functionCache[firFunction] = irFunction
+    }
+
     // ------------------------------------ constructors ------------------------------------
 
     @OptIn(IrSymbolInternals::class)

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/generators/DataClassMembersGenerator.kt

@@ -198,6 +198,7 @@ class DataClassMembersGenerator(val components: Fir2IrComponents) : Fir2IrCompon
                     components.irBuiltIns.stringType,
                 )
                 irClass.declarations.add(toStringFunction)
+                declarationStorage.cacheGeneratedFunction(toStringContributedFunction, toStringFunction)
             }
 
             val hashcodeNameContributedFunction = contributedSyntheticFunctions[HASHCODE_NAME]
@@ -209,6 +210,7 @@ class DataClassMembersGenerator(val components: Fir2IrComponents) : Fir2IrCompon
                     components.irBuiltIns.intType,
                 )
                 irClass.declarations.add(hashCodeFunction)
+                declarationStorage.cacheGeneratedFunction(hashcodeNameContributedFunction, hashCodeFunction)
             }
 
             val equalsContributedFunction = contributedSyntheticFunctions[EQUALS]
@@ -222,6 +224,7 @@ class DataClassMembersGenerator(val components: Fir2IrComponents) : Fir2IrCompon
                     isOperator = true
                 )
                 irClass.declarations.add(equalsFunction)
+                declarationStorage.cacheGeneratedFunction(equalsContributedFunction, equalsFunction)
             }
 
             return result
@@ -291,7 +294,7 @@ class DataClassMembersGenerator(val components: Fir2IrComponents) : Fir2IrCompon
             returnType: IrType,
             otherParameterNeeded: Boolean = false,
             isOperator: Boolean = false,
-        ): IrFunction {
+        ): IrSimpleFunction {
             val signature = if (klass.symbol.classId.isLocal) null else components.signatureComposer.composeSignature(syntheticCounterpart)
             return components.callablesGenerator.declareIrSimpleFunction(signature) { symbol ->
                 components.irFactory.createSimpleFunction(