gl_FragColor and gl_FragData are not available in Vulkan.  Prior to this
change, their declaration as webgl_FragColor and webgl_FragData was done
in text.  This change implements an AST transformation that declares a
normal fragment output variable and replaces all references to these
built-ins with those variables.

Bug: angleproject:4889
Change-Id: If224e089dec25e4aa580beb135e1be2890de7887
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2953042Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>