EXT_shader_framebuffer_fetch_non_coherent is implemented using subpass
input attachments. The extension will be enabled in a follow up change
that adds required changes to the Vulkan translator.

Bug: angleproject:5454
Test: FramebufferFetchNonCoherentES31.*Vulkan
Change-Id: Ic73c66a476c4a21db5269431166a198841f1dc0c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2598059
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>