Vulkan: Move xfb buffer decl to translator in emulation path 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>