glsl: Store a predicate for whether a built-in signature is available.

For the upcoming built-in function rewrite, we'll need to be able to
answer "Is this built-in function signature available?".

This is actually a somewhat complex question, since it depends on the
language version, GLSL vs. GLSL ES, enabled extensions, and the current
shader stage.

Storing such a set of constraints in a structure would be painful, so
instead we store a function pointer.  When creating a signature, we
simply point to a predicate that inspects _mesa_glsl_parse_state and
answers whether the signature is available in the current shader.

Unfortunately, IR reader doesn't actually know when built-in functions
are available, so this patch makes it lie and say that they're always
present.  This allows us to hook up the new functionality; it just won't
be useful until real data is populated.  In the meantime, the existing
profile mechanism ensures built-ins are available in the right places.

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

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