glsl: Always search for an exact function signature match.

glsl: Always search for an exact function signature match.

Previously, we would fail to compile the following shader due to a bug
in lazy built-in importing:

    #version 130
    void main() {
        float f = abs(5.0);
        int i = abs(5);
    }

The first call, abs(5.0), would fail to find a local signature, look
through the built-ins, and import "float abs(float)".

The second call, abs(5), would find the newly imported float signature
in the local shader, and settle for that.  Unfortunately, it failed to
search the built-ins for the correct/exact signature, "int abs(int)".

Thus, abs(5) ended up being a float, causing a bizarre type error when
we tried to assign it to an int.

Fixes piglit test builtin-overload-matching.frag.

This is /not/ a candidate for stable branches, as it should only be
possible to trigger this bug using GLSL 1.30's built-in functions that
take integer arguments.  Plus, the changes are fairly invasive.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>