This change also moves the ShaderType enum from D3D renderer to angletype.h.
And it uses a bit mask to track the linked shader stages.

BUG=angleproject:2247

Change-Id: I5c7ee1445d353a02e24549ffcf6b0ac694dd1069
Reviewed-on: https://chromium-review.googlesource.com/768629
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>