Add index() to LocalSignature for more clear access

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

@@ -194,6 +194,8 @@ sealed class IdSignature {
         override val isLocal: Boolean
             get() = true
 
+        fun index(): Int = hashSig?.toInt() ?: error("Expected index in ${render()}")
+
         override fun topLevelSignature(): IdSignature {
             error("Illegal access: Local Sig does not have toplevel (${render()}")
         }

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DescriptorByIdSignatureFinder.kt

@@ -57,7 +57,7 @@ class DescriptorByIdSignatureFinder(
                 fqn == MangleConstant.TYPE_PARAMETER_MARKER_NAME || fqn == MangleConstant.TYPE_PARAMETER_MARKER_NAME_SETTER
 
             if (isTypeParameterSig(inner.localFqn)) {
-                val tpIndex = inner.hashSig?.toInt() ?: error("Expected index in $signature")
+                val tpIndex = inner.index()
                 if (container is CallableDescriptor) {
                     return container.typeParameters[tpIndex]
                 }

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/IrModuleDeserializer.kt

@@ -155,8 +155,8 @@ class IrModuleDeserializerWithBuiltIns(
             val classSignature = idSig.container
             val classSymbol = resolveFunctionalInterface(classSignature, BinarySymbolData.SymbolKind.CLASS_SYMBOL) as IrClassSymbol
             val typeParameterSig = composite.inner as IdSignature.LocalSignature
-            val typeParameterIndex = typeParameterSig.hashSig ?: error("Expected index for $idSig")
-            val typeParameter = classSymbol.owner.typeParameters[typeParameterIndex.toInt()]
+            val typeParameterIndex = typeParameterSig.index()
+            val typeParameter = classSymbol.owner.typeParameters[typeParameterIndex]
             return typeParameter.symbol
         }
         val publicSig = idSig.asPublic() ?: error("$idSig has to be public")