Fix false positive "Null can not be a value of a non-null type"

Inferred type of receiver of orElse  is Optional<T & Any>
Generic descriptor is orElse(E!): E!
Substituted descriptor is orElse(T): T , and that is the problem.

Seems that E!  => (T & Any)!  gets expanded to just T  or T & Any , however it should be expanded to
(T & Any) .. (T & Any)?  => T & Any .. T & Any

T & Any is NotNullTypeParameter(T)

The problem is that (T & Any)?  is expanded to T & Any,
that is seems to be wrong.

 #KT-15236 Fixed

compiler/testData/diagnostics/testsWithJava8/nullForOptionalOrElse.kt

@@ -0,0 +1,7 @@
+import java.util.*
+import java.util.stream.Stream
+
+fun <T> Stream<T>?.getIfSingle() =
+        this?.map { Optional.ofNullable(it) }
+        ?.reduce(Optional.empty()) { _, _ -> Optional.empty() }
+        ?.orElse(null) // <<---- should not be an error

compiler/testData/diagnostics/testsWithJava8/nullForOptionalOrElse.txt

@@ -0,0 +1,3 @@
+package
+
+public fun </*0*/ T> java.util.stream.Stream<T>?.getIfSingle(): T?

compiler/tests-java8/tests/org/jetbrains/kotlin/checkers/DiagnosticsWithJava8TestGenerated.java

@@ -48,6 +48,12 @@ public class DiagnosticsWithJava8TestGenerated extends AbstractDiagnosticsWithFu
         doTest(fileName);
     }
 
+    @TestMetadata("nullForOptionalOrElse.kt")
+    public void testNullForOptionalOrElse() throws Exception {
+        String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/testsWithJava8/nullForOptionalOrElse.kt");
+        doTest(fileName);
+    }
+
     @TestMetadata("samWithConsumer.kt")
     public void testSamWithConsumer() throws Exception {
         String fileName = KotlinTestUtils.navigationMetadata("compiler/testData/diagnostics/testsWithJava8/samWithConsumer.kt");

core/descriptor.loader.java/src/org/jetbrains/kotlin/load/java/typeEnhancement/typeEnhancement.kt

@@ -243,7 +243,8 @@ internal class NotNullTypeParameter(override val delegate: SimpleType) : CustomT
     }
 
     override fun replaceAnnotations(newAnnotations: Annotations) = NotNullTypeParameter(delegate.replaceAnnotations(newAnnotations))
-    override fun makeNullableAsSpecified(newNullability: Boolean) = this
+    override fun makeNullableAsSpecified(newNullability: Boolean) =
+        if (newNullability) delegate.makeNullableAsSpecified(true) else this
 
     override val isError: Boolean
         get() = false