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/ir/irText/expressions/sam/samConversionToGeneric.fir.kt.txt

@@ -1,8 +1,8 @@
-fun test1(): J<String> {
+fun test1(): J<@FlexibleNullability String?> {
   return local fun <anonymous>(x: @FlexibleNullability String?): @FlexibleNullability String? {
     return x
   }
- /*-> J<String> */
+ /*-> J<@FlexibleNullability String?> */
 }
 
 fun test10(fn: Function1<Int, String>) {
@@ -49,4 +49,3 @@ fun test8(efn: @ExtensionFunctionType Function1<String, String>): J<@FlexibleNul
 fun test9(efn: @ExtensionFunctionType Function1<String, String>) {
   bar<@FlexibleNullability String?>(j = efn /*-> J<@FlexibleNullability String?> */)
 }
-