Follow-on CL needs the ProgramMergedVaryings in the Vulkan backend to
generate valid SPIRV.

Bug: angleproject:3078
Change-Id: Ic442a3e0bd713fec36bd6b9420f67f3b1118e5ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2404336Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>