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/java8Tests/jspecify/strictMode/interconnectedGenerics.fir.txt

@@ -34,7 +34,7 @@ FILE: Eq.kt
             public get(): R|lib/a/A<R, S>|
 
         public open override fun withL(l: R|kotlin/Int|): R|lib/a/C.WithL<R, S>| {
-            ^withL Q|lib/a/C|.R|lib/a/C.MyWithL.MyWithL|<R|R|, R|S|>()
+            ^withL Q|lib/a/C|.R|lib/a/C.MyWithL.MyWithL|<R|R!|, R|S!|>()
         }
 
     }

compiler/testData/diagnostics/foreignAnnotationsTests/java8Tests/misc/returnType.fir.kt

@@ -37,7 +37,7 @@ fun <R> main(a: ReturnType<R>) {
     takeNullableStringAndKNullable(x1)
     takeNotNullStringAndNotNullK(<!ARGUMENT_TYPE_MISMATCH!>x1<!>)
     takeNullableStringAndNotNullK(<!ARGUMENT_TYPE_MISMATCH!>x1<!>)
-    takeNotNullString(a.foo41.foo411)
+    takeNotNullString(<!ARGUMENT_TYPE_MISMATCH!>a.foo41.foo411<!>)
 
     val x2 = <!DEBUG_INFO_EXPRESSION_TYPE("ReturnType.A<kotlin.String?, R!!>..ReturnType.A<kotlin.String?, R!!>?!")!>a.foo2()<!>
     takeNotNullStringAndKNullable(<!ARGUMENT_TYPE_MISMATCH!>x2<!>)

compiler/testData/diagnostics/foreignAnnotationsTests/java8Tests/misc/returnTypeWithWarnings.fir.kt

@@ -39,7 +39,7 @@ fun <R> main(a: ReturnTypeWithWarnings<R>) {
     takeNullableStringAndKNullable(x1)
     takeNotNullStringAndNotNullK(<!ARGUMENT_TYPE_MISMATCH!>x1<!>)
     takeNullableStringAndNotNullK(<!ARGUMENT_TYPE_MISMATCH!>x1<!>)
-    takeNotNullString(a.foo41.foo411)
+    takeNotNullString(<!ARGUMENT_TYPE_MISMATCH!>a.foo41.foo411<!>)
 
     val x2 = <!DEBUG_INFO_EXPRESSION_TYPE("ReturnTypeWithWarnings.A<kotlin.String?, R!!>..ReturnTypeWithWarnings.A<kotlin.String?, R!!>?!")!>a.foo2()<!>
     takeNotNullStringAndKNullable(<!ARGUMENT_TYPE_MISMATCH!>x2<!>)