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/codegen/bytecodeListing/nullabilityAnnotations/platformTypes.kt

@@ -1,8 +1,18 @@
 // WITH_RUNTIME
+// FILE: test.kt
+
 import java.util.Collections
 
 class PlatformTypes {
     fun simplyPlatform() = Collections.singletonList("")[0]
     fun bothNullable() = Collections.emptyList<String>() ?: null
     fun bothNotNull() = Collections.emptyList<String>()!!
+
+    fun <R> generic(j: J<R>) = j.get()
+}
+
+// FILE: J.java
+
+public interface J<R> {
+    R get();
 }

compiler/testData/codegen/bytecodeListing/nullabilityAnnotations/platformTypes.txt

@@ -1,8 +1,9 @@
 @kotlin.Metadata
 public final class PlatformTypes {
-    // source: 'platformTypes.kt'
+    // source: 'test.kt'
     public method <init>(): void
     public final @org.jetbrains.annotations.NotNull method bothNotNull(): java.util.List
     public final @org.jetbrains.annotations.Nullable method bothNullable(): java.util.List
+    public final method generic(@org.jetbrains.annotations.NotNull p0: J): java.lang.Object
     public final method simplyPlatform(): java.lang.String
 }