Fragment shader built-ins are implemented in this change and direct
SPIR-V generation for fragment shaders is enabled.

Bug: angleproject:4889
Change-Id: I6f92a5585f242122a81c97a9b1aa2763009161a0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2951625
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>