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>

src/glsl/ir_clone.cpp

@@ -330,6 +330,7 @@ ir_function_signature::clone_prototype(void *mem_ctx, struct hash_table *ht) con
 
    copy->is_defined = false;
    copy->is_builtin = this->is_builtin;
+   copy->builtin_info = this->builtin_info;
    copy->origin = this;
 
    /* Clone the parameter list, but NOT the body.