JVM IR: implement MethodSignatureMapper.mapFunctionName directly

compiler/testData/codegen/boxInline/reified/kt18977.kt

@@ -1,5 +1,5 @@
-// IGNORE_BACKEND_MULTI_MODULE: JVM_IR
 // FILE: 1.kt
+
 package test
 
 class AbstractTreeNode<T>(val value: T, val parent: AbstractTreeNode<T>?)
@@ -24,4 +24,4 @@ import test.*
 
 fun box(): String {
     return AbstractTreeNode("OK", null).findNotNullValueOfType<String>()!!
-}
+}