Refactor KotlinType::asPsiType for ultra-light classes

- Extract HOF that UltraLightSupport::mapType that allows to call
arbitrary functions of type mapper
- Use it for computing return type of functions
- "declaration: KtDeclaration" parameter became unused since it was only
used for return types

compiler/backend/src/org/jetbrains/kotlin/codegen/state/KotlinTypeMapper.java

@@ -388,7 +388,7 @@ public class KotlinTypeMapper {
     }
 
     @NotNull
-    private Type mapReturnType(@NotNull CallableDescriptor descriptor, @Nullable JvmSignatureWriter sw) {
+    public Type mapReturnType(@NotNull CallableDescriptor descriptor, @Nullable JvmSignatureWriter sw) {
         KotlinType returnType = descriptor.getReturnType();
         assert returnType != null : "Function has no return type: " + descriptor;