Support creating flexible types in tests

Flexible types can not be created in normal user code, but if there's a special classifier `kotlin.internal.flexible.ft` with two type parameters,
its occurrences are replaced by flexible types, e.g. `ft<Int, Int?>` becomes `Int!`

compiler/testData/diagnostics/tests/platformTypes/explicitFlexibleNoPackage.kt

@@ -0,0 +1,8 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// !EXPLICIT_FLEXIBLE_TYPES
+// !CHECK_TYPE
+
+fun foo(f: ft<Int, Int?>) {
+    f.checkType { it : _<Int> }
+    f.checkType { it : _<Int?> }
+}

compiler/testData/diagnostics/tests/platformTypes/explicitFlexibleNoPackage.txt

@@ -0,0 +1,11 @@
+package
+
+internal fun foo(/*0*/ f: kotlin.Int!): kotlin.Unit
+internal fun </*0*/ T> T.checkType(/*0*/ f: (_<T>) -> kotlin.Unit): (_<T>) -> kotlin.Unit
+
+internal final class _</*0*/ T> {
+    public constructor _</*0*/ T>()
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}

compiler/testData/diagnostics/tests/platformTypes/explicitFlexibleWithPackage.kt

@@ -0,0 +1,9 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+// !EXPLICIT_FLEXIBLE_TYPES
+// !CHECK_TYPE
+package ppp
+
+fun foo(f: ft<Int, Int?>) {
+    f.checkType { it : _<Int> }
+    f.checkType { it : _<Int?> }
+}

compiler/testData/diagnostics/tests/platformTypes/explicitFlexibleWithPackage.txt

@@ -0,0 +1,13 @@
+package
+
+package ppp {
+    internal fun foo(/*0*/ f: kotlin.Int!): kotlin.Unit
+    internal fun </*0*/ T> T.checkType(/*0*/ f: (ppp._<T>) -> kotlin.Unit): (ppp._<T>) -> kotlin.Unit
+
+    internal final class _</*0*/ T> {
+        public constructor _</*0*/ T>()
+        public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+        public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+        public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+    }
+}