encoding/openapi: support time types

Also recognize unsupported builtins for
their type.

Issue #56

Change-Id: Ic900829144d0130db412c1202f82df4e3a56c236
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2724
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/openapi_test.go b/encoding/openapi/openapi_test.go
index 9fbe314..bcdaf19 100644
--- a/encoding/openapi/openapi_test.go
+++ b/encoding/openapi/openapi_test.go
@@ -63,6 +63,10 @@
 		"nums.json",
 		defaultConfig,
 	}, {
+		"builtins.cue",
+		"builtins.json",
+		defaultConfig,
+	}, {
 		"oneof.cue",
 		"oneof.json",
 		defaultConfig,