Check isSwiftShader() for whether to load swiftshader instead of the
default ICD.

The spec says we should query GL_DEVICE_UUID_EXT & co for this, but we
haven't implemented those queries yet.

Bug: angleproject:4092
Change-Id: Ifd74d6b6706f7b1fcbe5d82a33ecf70e631e26b9
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2088660Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>