gs: Test redeclaring either gl_PerVertex in or out, and using both.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Test a corner case involving arrays, structs, and interface blocks.

This patch exercises the same corner case as
https://bugs.freedesktop.org/show_bug.cgi?id=70368.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>