This allows removal of isPerVertexBuiltIn(). It also leads to
removal of addInterstageIoToLinkage(), which is no longer needed.
Includes related name improvements.
The goal is to flatten all I/O, but there are multiple categories and
steps to complete, likely including a final unification of splitting
and flattening.
In the hull shader, the PCF output does not participate in an argument list,
so has no defined ordering. It is always put at the end of the linkage. That
means the DS input reading PCF data must be be at the end of the DS linkage
as well, no matter where it may appear in the argument list. This change
makes sure that happens.
The detection is by looking for arguments that contain tessellation factor
builtins, even as a struct member. The whole struct is taken as the PCF output
if any members are so qualified.
