[FIR2IR] Don't use descriptor manglers in FIR2IR

They don't do anything, only complicate the initial configuration.

KT-64990

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/DescriptorSymbolTableExtension.kt

@@ -38,7 +38,7 @@ open class DescriptorSymbolTableExtension(table: SymbolTable) : SymbolTableExten
         get() = table.nameProvider
 
     private val signatureComposer: IdSignatureComposer
-        get() = table.signaturer
+        get() = table.signaturer!!
 
     private val externalPackageFragmentSlice: SymbolTableSlice<PackageFragmentDescriptor, IrExternalPackageFragment, IrExternalPackageFragmentSymbol> = SymbolTableSlice.Flat(lock)
 

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/util/SymbolTable.kt

@@ -17,7 +17,7 @@ private fun <SymbolOwner : IrSymbolOwner, Symbol : IrBindableSymbol<*, SymbolOwn
 
 @OptIn(SymbolTableInternals::class)
 open class SymbolTable(
-    val signaturer: IdSignatureComposer,
+    val signaturer: IdSignatureComposer?, // TODO: This is `null` only in FIR2IR. Make non-nullable again after KT-64990 is fixed.
     val irFactory: IrFactory,
     val nameProvider: NameProvider = NameProvider.DEFAULT,
 ) : ReferenceSymbolTable {