[wgsl] Functions section defines WGSL builtins as always being available.

[dj2]

In the Functions section of the spec we say: A built-in function is provided by the WGSL implementation, and is always available to a WGSL module. While this was true historically, it is no longer technically true.

If I've shadowed one of the builtin names, then that builtin is no longer available to be used. So, the builtin is only available if it is not shadowed. This will change after we have namespaces but it would be useful to clarify that portion of the spec and we can update it when/if namespaces land.