Fix KotlinType.isNullabilityFlexible for types based on type parameters

In the compiler, this function was used in psi2ir (and it affected
nullability annotations in the resulting JVM class files), in "useless
elvis with null on the right side" diagnostic checker, and in light
classes.

 #KT-42650 Fixed

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.fir.kt

@@ -24,6 +24,12 @@ public class JJJ {
     public static JJJ staticNNN;
 }
 
+// FILE: JJJJ.java
+
+public interface JJJJ<R> {
+    R get();
+}
+
 // FILE: k.kt
 
 fun test() {
@@ -37,3 +43,5 @@ fun test() {
 
 fun foo(a: Any?) {
 }
+
+fun <R> test2(j: JJJJ<R>) = j.get() ?: null

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.kt

@@ -24,6 +24,12 @@ public class JJJ {
     public static JJJ staticNNN;
 }
 
+// FILE: JJJJ.java
+
+public interface JJJJ<R> {
+    R get();
+}
+
 // FILE: k.kt
 
 fun test() {
@@ -37,3 +43,5 @@ fun test() {
 
 fun foo(a: Any?) {
 }
+
+fun <R> test2(j: JJJJ<R>) = j.get() ?: null

compiler/testData/diagnostics/tests/platformTypes/nullabilityWarnings/uselessElvisRightIsNull.txt

@@ -2,6 +2,7 @@ package
 
 public fun foo(/*0*/ a: kotlin.Any?): kotlin.Unit
 public fun test(): kotlin.Unit
+public fun </*0*/ R> test2(/*0*/ j: JJJJ<R>): R?
 
 public open class J {
     public constructor J()
@@ -32,3 +33,10 @@ public open class JJJ {
     // Static members
     @org.jetbrains.annotations.NotNull public final var staticNNN: JJJ
 }
+
+public interface JJJJ</*0*/ R : kotlin.Any!> {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public abstract fun get(): R!
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}