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/foreignAnnotationsTests/tests/jsr305/nullabilityWarnings/override.fir.kt

@@ -66,10 +66,10 @@ abstract class A<T> : Provider<T> {
 abstract class B<T> : Provider<T> {
     override fun get(): T & Any = null!!
     override fun getNullable(): T? = null!!
-    override fun set(x: T & Any) {} // False positive in K1
+    <!NOTHING_TO_OVERRIDE!>override<!> fun set(x: T & Any) {} // False positive in K1
     override fun setNullable(x: T?) {}
 
-    override fun getSet(x: T & Any): T & Any = x // False positive in K1
+    <!NOTHING_TO_OVERRIDE!>override<!> fun getSet(x: T & Any): T & Any = x // False positive in K1
     override fun getSetNullable(x: T?): T? = x
 }
 
@@ -79,6 +79,6 @@ abstract class C<T> : Provider<T> {
 }
 
 abstract class D<T> : Provider<T> {
-    <!WRONG_NULLABILITY_FOR_JAVA_OVERRIDE!>override<!> fun getSet(x: T & Any): T = x<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!> // False positive in K1
+    <!NOTHING_TO_OVERRIDE!>override<!> fun getSet(x: T & Any): T = x<!UNNECESSARY_NOT_NULL_ASSERTION!>!!<!> // False positive in K1
     override fun getSetNullable(x: T?): T = x!!
 }