If not declared by the shader, default gl_PerVertex is now explicitly
declared in AST prior to SPIR-V generation.  The SPIR-V generation code
then doesn't need to declare them magically.

Bug: angleproject:4889
Change-Id: Icc593bc1ccc3162487bdbae7f66bc775d098778d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2905952
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>