Currently `_LIBCPP_OVERRIDABLE_FUNCTION` takes the return type, function
name and argument list, but simply constructs the function and adds
attributes without modifying the signature in any way. We can replace
this with a normal attribute macro, making the signature easier to read
and simpler to understand what's actually going on. Since it's an
internal macro we can also drop the `_LIBCPP_` prefix.
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