glsl: Move is_builtin flag back to ir_function_signature.

This effectively reverts b6f15869b324ae64a00d0fe46fa3c8c62c1edb6c.

In desktop GLSL, defining a function with the same name as a built-in
hides that built-in function completely, so there would never be
built-in and user function signatures in the same ir_function.

However, in GLSL ES, overloading built-ins is allowed, and does not
hide the built-in signatures - so we're back to needing this.

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