[FIR2IR] Load builtin classes instead of creating them on the fly

This change uncovered KT-61282, which was hidden because of incorrect
  module fragments used as parent for builtin classes

compiler/fir/cones/src/org/jetbrains/kotlin/fir/types/ConeBuiltinTypeUtils.kt

@@ -31,6 +31,7 @@ val ConeKotlinType.isThrowableOrNullableThrowable: Boolean get() = isAnyOfBuilti
 val ConeKotlinType.isChar: Boolean get() = isBuiltinType(StandardClassIds.Char, false)
 val ConeKotlinType.isCharOrNullableChar: Boolean get() = isAnyOfBuiltinType(setOf(StandardClassIds.Char))
 val ConeKotlinType.isString: Boolean get() = isBuiltinType(StandardClassIds.String, false)
+val ConeKotlinType.isNullableString: Boolean get() = isBuiltinType(StandardClassIds.String, true)
 
 val ConeKotlinType.isEnum: Boolean get() = isBuiltinType(StandardClassIds.Enum, false)
 

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrConverter.kt

@@ -564,8 +564,7 @@ class Fir2IrConverter(
             components.visibilityConverter = visibilityConverter
             components.typeConverter = Fir2IrTypeConverter(components)
             val irBuiltIns = initializedIrBuiltIns ?: IrBuiltInsOverFir(
-                components, fir2IrConfiguration.languageVersionSettings, moduleDescriptor, irMangler,
-                fir2IrConfiguration.languageVersionSettings.getFlag(AnalysisFlags.builtInsFromSources) || kotlinBuiltIns !== DefaultBuiltIns.Instance
+                components, fir2IrConfiguration.languageVersionSettings, moduleDescriptor, irMangler
             )
             components.irBuiltIns = irBuiltIns
             val conversionScope = Fir2IrConversionScope()
@@ -578,6 +577,7 @@ class Fir2IrConverter(
             components.annotationsFromPluginRegistrar = Fir2IrAnnotationsFromPluginRegistrar(components)
 
             fir2IrExtensions.registerDeclarations(commonMemberStorage.symbolTable)
+            irBuiltIns.initialize()
 
             val irModuleFragment = IrModuleFragmentImpl(moduleDescriptor, irBuiltIns)