GLSL allows varyings passed from one stage to another to not match in
precision (e.g. float & half-float). Vulkan doesn't allow that so adjust
those mismatches to use the higher precision.

To fix we keep track of the precision of varyings and in the Vulkan
backend if we see they are different patch up the SPIR-V to make them
match.

Bug: angleproject:3078
Change-Id: I385d31e082da46ccdd4817b6612f5f9d9cbce17c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2337755
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>