[FIR2IR] Introduce & use FirBuiltInsPackageFragment

Without this commit, JVM name mapping logic in BE does not work for FIR,
because FIR cannot use old BuiltInsPackageFragmentImpl descriptor.
In this commit we add our own implementation thus fixing
a pack of FIR black box tests.

compiler/testData/codegen/box/classes/kt343.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KJS_WITH_FULL_RUNTIME
 fun launch(f : () -> Unit) {
     f()

compiler/testData/codegen/box/classes/overloadBinaryOperator.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KJS_WITH_FULL_RUNTIME
 class ArrayWrapper<T>() {
     val contents = ArrayList<T>()

compiler/testData/codegen/box/classes/overloadPlusAssign.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KJS_WITH_FULL_RUNTIME
 class ArrayWrapper<T>() {
     val contents = ArrayList<T>()

compiler/testData/codegen/box/classes/overloadPlusAssignReturn.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KJS_WITH_FULL_RUNTIME
 class ArrayWrapper<T>() {
     val contents = ArrayList<T>()

compiler/testData/codegen/box/classes/overloadPlusToPlusAssign.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KJS_WITH_FULL_RUNTIME
 class ArrayWrapper<T>() {
     val contents = ArrayList<T>()