glsl: stop cloning builtin fuctions _mesa_glsl_find_builtin_function()

The cloning was introduced in f81ede469910d to fix a problem with
shaders including IR that was owned by builtins.

However the approach of cloning the whole function each time we
reference a builtin lead to a significant reduction in the GLSL
IR compilers performance.

The previous patch fixes the ownership problem in a more precise
way. So we can now remove this cloning.

Testing on a Ryzen 7 1800X shows a ~15% decreases in compiling the
Deus Ex: Mankind Divided shaders on radeonsi (which take 5min+ on
some machines). Looking just at the GLSL IR compiler the speed up
is ~40%.

Tested-by: Dieter Nützel <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>
Reviewed-by: Nicolai Hähnle <[email protected]>

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