glsl: Make it possible to ignore built-ins when matching signatures.

glsl: Make it possible to ignore built-ins when matching signatures.

Historically, we've implemented the rules for overriding built-in
functions by creating multiple ir_functions and relying on the symbol
table to hide the one containing built-in functions.  That works, but
has a few drawbacks, so the next patch will change it.

Instead, we'll have a single ir_function for a particular name, which
will contain both built-in and user-defined signatures.  Passing an
extra parameter to matching_signature makes it easy to ignore built-ins
when they're supposed to be hidden.

I didn't add the parameter to exact_matching_signature since it wasn't
necessary.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>