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/firProblems/AllCandidates.fir.kt.txt

@@ -21,7 +21,7 @@ class ResolvedCall<C : Any?> {
 }
 
 private fun <A : Any?> allCandidatesResult(allCandidates: Collection<MyCandidate>): @FlexibleNullability OverloadResolutionResultsImpl<@FlexibleNullability A?>? {
-  return nameNotFound<A>().apply<@FlexibleNullability OverloadResolutionResultsImpl<@FlexibleNullability A?>?>(block = local fun @FlexibleNullability OverloadResolutionResultsImpl<@FlexibleNullability A?>?.<anonymous>() {
+  return nameNotFound<@FlexibleNullability A?>().apply<@FlexibleNullability OverloadResolutionResultsImpl<@FlexibleNullability A?>?>(block = local fun @FlexibleNullability OverloadResolutionResultsImpl<@FlexibleNullability A?>?.<anonymous>() {
     $this$apply.setAllCandidates(allCandidates = allCandidates.map<MyCandidate, ResolvedCall<A>>(transform = local fun <anonymous>(it: MyCandidate): ResolvedCall<A> {
       return it.<get-resolvedCall>() as ResolvedCall<A>
     }