Propagate nullability changes to enhancement for JSR-305 types

Fix TypeUtils.makeNullableAsSpecified for SimpleTypeWithEnhancement and
FlexibleTypeWithEnhancement: change nullability of enhancement too. This
fixes several false-positive warnings, like in KT-20855 and KT-20466.

Note that it removes warning in some cases (see testdata change for
uselessElvisRightIsNull.kt). However, this removes warning about
*unnecessary* elvis, i.e. this fixed introduces weak false-negatives, which
is acceptable for the moment.

#KT-20855 Fixed Target versions 1.2.30
#KT-20466 Fixed Target versions 1.2.30
#KT-21238 Fixed Target versions 1.2.30

core/descriptors/src/org/jetbrains/kotlin/types/TypeWithEnhancement.kt

@@ -38,7 +38,7 @@ class SimpleTypeWithEnhancement(
             = origin.replaceAnnotations(newAnnotations).wrapEnhancement(enhancement) as SimpleType
 
     override fun makeNullableAsSpecified(newNullability: Boolean): SimpleType
-            = origin.makeNullableAsSpecified(newNullability).wrapEnhancement(enhancement) as SimpleType
+            = origin.makeNullableAsSpecified(newNullability).wrapEnhancement(enhancement.unwrap().makeNullableAsSpecified(newNullability)) as SimpleType
 }
 
 class FlexibleTypeWithEnhancement(
@@ -51,7 +51,7 @@ class FlexibleTypeWithEnhancement(
             = origin.replaceAnnotations(newAnnotations).wrapEnhancement(enhancement)
 
     override fun makeNullableAsSpecified(newNullability: Boolean): UnwrappedType
-            = origin.makeNullableAsSpecified(newNullability).wrapEnhancement(enhancement)
+            = origin.makeNullableAsSpecified(newNullability).wrapEnhancement(enhancement.unwrap().makeNullableAsSpecified(newNullability))
 
     override fun render(renderer: DescriptorRenderer, options: DescriptorRendererOptions): String
             = origin.render(renderer, options)