Add util method to BuildIns is kotlin.Function type.

core/descriptors/src/org/jetbrains/kotlin/builtins/KotlinBuiltIns.java

@@ -275,6 +275,8 @@ public abstract class KotlinBuiltIns {
 
         public final FqNameUnsafe _enum = fqNameUnsafe("Enum");
 
+        public final FqNameUnsafe functionSupertype = fqNameUnsafe("Function");
+
 
 
         public final FqName throwable = fqName("Throwable");
@@ -1094,6 +1096,10 @@ public abstract class KotlinBuiltIns {
         return false;
     }
 
+    public static boolean isNotNullOrNullableFunctionSupertype(@NotNull KotlinType type) {
+        return isConstructedFromGivenClass(type, FQ_NAMES.functionSupertype);
+    }
+
     public static FqName getPrimitiveFqName(@NotNull PrimitiveType primitiveType) {
         return BUILT_INS_PACKAGE_FQ_NAME.child(primitiveType.getTypeName());
     }