K2: repeat K1 representation for flexible type parameters

This commit changes the behavior of KT-59138 effectively declining it in 2.0.
However, we plan to implement KT-59138 behavior under a feature
flag in 2.0 (see KT-66447), and switch this feature on version 2.x.

Also, this commit implements the LC resolution about postponing
KT-57014 change. We don't have KT-57014 described behavior in 2.0 anymore.
However, we plan to implement a deprecation warning here, see KT-65578.

After this commit, 6 diagnostic tests become incorrectly broken:
- 5 tests from PurelyImplementedCollection group
- a test platformTypes/nullableTypeArgument.kt

This commit also breaks currently fixed-in-k2 KT-50134
(it is fixed again in the following commits),
as well as KT-58933 (it will remain not fixed till we enable KT-59138
behavior again).

#KT-65596 In Progress
#KT-57014 In Progress
#KT-58933 Submitted

compiler/testData/diagnostics/tests/explicitDefinitelyNotNullableViaIntersection/overridesJavaAnnotated.fir.kt

@@ -1,42 +0,0 @@
-// !LANGUAGE: +DefinitelyNonNullableTypes +ProhibitUsingNullableTypeParameterAgainstNotNullAnnotated
-
-// FILE: A.java
-import org.jetbrains.annotations.*;
-
-public interface A<T> {
-    public T foo(T x) { return x; }
-    @NotNull
-    public T bar(@NotNull T x) {}
-}
-
-// FILE: main.kt
-
-interface B<T1> : A<T1> {
-    override fun foo(x: T1): T1
-    override fun bar(x: T1 & Any): T1 & Any
-}
-
-interface C<T2> : A<T2> {
-    override fun foo(x: T2 & Any): T2 & Any
-    <!NOTHING_TO_OVERRIDE!>override<!> fun bar(x: T2): T2
-}
-
-interface D : A<String?> {
-    override fun foo(x: String?): String?
-    override fun bar(x: String): String
-}
-
-interface E : A<String> {
-    override fun foo(x: String): String
-    override fun bar(x: String): String
-}
-
-interface F : A<String?> {
-    override fun foo(x: String): String
-    <!NOTHING_TO_OVERRIDE!>override<!> fun bar(x: String?): String?
-}
-
-interface G<T3 : Any> : A<T3> {
-    override fun foo(x: T3): T3
-    override fun bar(x: T3): T3
-}

compiler/testData/diagnostics/tests/explicitDefinitelyNotNullableViaIntersection/overridesJavaAnnotated.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !LANGUAGE: +DefinitelyNonNullableTypes +ProhibitUsingNullableTypeParameterAgainstNotNullAnnotated
 
 // FILE: A.java