With specialization constant qualifier and ids set in the node types,
the SPIR-V generation for these variables are trivially handled.

This is in preparation for enabling line raster emulation as well as
fragment shaders in general.

Bug: angleproject:4889
Change-Id: Ie4d4e47b222ac66d1e0eee29f55ac09990313580
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2939337
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>