[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/extensionFunctions/kt1953.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // KJS_WITH_FULL_RUNTIME
 fun box(): String {
     val sb = StringBuilder()

compiler/testData/codegen/box/extensionFunctions/kt5467.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun String.foo() : String {
     fun Int.bar() : String {
         fun Long.baz() : String {