[FIR2IR] Introduce staged transformation (first step)

Now FE IR -> BE IR transformation is performed in multiple stages
controller by Fir2IrConverter. Stages are
 * files & classes registration
 * supertypes & type parameters handling
 * functions & properties signature generation
 * body generation

After each step we have guarantee (with exception of local classes &
type inference combination, and external symbols) that required symbols
(class/function/property/variable/type parameter)
are already bound to real declarations and have correct parents.

This commit also fixes incorrect parents for local classes

compiler/testData/codegen/box/ir/closureConversion/closureConversion1.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun foo(x: String): String {
     fun bar(y: String) = x + y
     return bar("K")

compiler/testData/codegen/box/ir/closureConversion/closureConversion2.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun <X : Any> foo(x: X): String {
     fun <Y : Any> bar(y: Y) =
             x.toString() + y.toString()

compiler/testData/codegen/box/ir/closureConversion/closureConversion3.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun foo(x: String): String {
     fun bar(y: String): String {
         fun qux(z: String): String =

compiler/testData/codegen/box/ir/closureConversion/closureConversion4.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun String.foo(): String {
     fun bar(y: String) = this + y
     return bar("K")

compiler/testData/codegen/box/ir/closureConversion/mutable1.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box(): String {
     var result = ""
     fun add(s: String) {