Shadow samplers use a different sampler object type in D3D11 HLSL than
other ESSL samplers. They need to be able to coexist in the same shader.

BUG=angleproject:1261
TEST=angle_end2end_tests

Change-Id: I960f6a41c0237ed912b072527e14aebeca98cf7c
Reviewed-on: https://chromium-review.googlesource.com/322120
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>