This makes @@ XFB-DECL @@ empty on this path.  Ultimately, this is
working towards removing both @@ XFB-DECL @@ and @@ XFB-OUT @@ macros
for both the emulation and extension paths, allowing the shaders to be
compiled at compile time rather than link time.

Bug: angleproject:3606
Change-Id: If16e9d92c419a04ecd3094481ed546d0708cdb43
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2611305
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>