Add std::ostream output for packed enums. This can be used to convert them to strings for debugging. It can also be helpful for frame capture and replay. Currently the enums are output as GLenum values. Bug: angleproject:3611 Change-Id: Ifcd04449e0ef61e125e0db65aa0dcc2bf48b38ca Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1678399 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Commit-Queue: Jamie Madill <jmadill@chromium.org>