FIR2IR: introduce conversion scope, remove dispatch receiver for lambdas

compiler/testData/codegen/box/classes/kt1611.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box(): String {
     return Foo().doBar("OK")
 }

compiler/testData/codegen/box/classes/kt1976.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 class A {
     public val f : ()->String = {"OK"}
 }