Adds support for ES 3.0 shadow samplers.

Test: ./angle_deqp_gles3_no_gtest --deqp-egl-display-type=angle-vulkan -n 'dEQP-GLES3.functional.texture.shadow.2d.*'

Bug: angleproject:3211
Change-Id: Ic82ecfe19290fb952fcb0ba423691b5bac87c4a3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1652021Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>