[FIR2IR] Generate fake overrides earlier and bind them later

Before this commit, we generated fake overrides at last FIR2IR stage,
after having all functions and classes built. This could lead to a
situation when fake override was called before it was generated.
This commit fixes this situation.

compiler/testData/codegen/box/classes/kt2480.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 fun box() = Class().printSome()
 
 public abstract class AbstractClass<T> {