When linking program varyings, the necessary functions need the
following:
- Lists of Input and Output varyings
- Shader versions
- Shader types

Rather than the full Shader itself, just this information will be
passed around to the functions. This allows us to limit how much data
is saved for each Program when linkProgram() succeeds, so PPos can link
the attached Programs at draw time.

Bug: angleproject:4514
Test: CQ
Change-Id: I27b639cae9a153d0d3f5efab3b38550f09f4f49b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2181449
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>