Addition of 'sample' qualifier keyword and 'interplateAt*'
fragment shader builtin functions with autogen

Bug: angleproject:3589
Change-Id: If358eb371fbcefffa715c8da4ba5e96eefaf6f52
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2477904
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>