During compilation, we'll use this to determine built-in availability.
The plan is to have a single shader containing every built-in in every
version of the language, but filter out the ones that aren't actually
available to the shader being compiled.

At link time, we don't actually need this filtering capability: we've
already imported prototypes for every built-in that the shader actually
calls, and they're flagged as is_builtin().  The linker doesn't import
any additional prototypes, so it won't pull in any unavailable
built-ins.  When resolving prototypes to function definitions, the
linker ensures the values of is_builtin() match, which means that a
shader can't trick the linker into importing the body of an unavailable
built-in by defining a suspiciously similar prototype.

In other words, during linking, we can just pass in NULL.  It will work
out fine.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>