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/tests-spec/testData/diagnostics/linked/expressions/conditional-expression/p-6/neg/1.1.fir.kt

@@ -62,8 +62,8 @@ public class JavaClassCase3{
 // TESTCASE NUMBER: 3
 fun case3() {
     val x = JavaClassCase3.id(null) // Nothing!
-    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing..kotlin.Nothing?!")!>x<!>
-    val a = if (x) {
+    <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Nothing?")!>x<!>
+    val a = if (<!CONDITION_TYPE_MISMATCH!>x<!>) {
         "NOK"
     } else "NOK"
 }