[libc++][NFC] Refactor _LIBCPP_OVERRIDABLE_FUNCTION to be a normal attribute macro (#174964)

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.

NOKEYCHECK=True
GitOrigin-RevId: a5cab90071fbf6f700c95c337d850845ccc9ed9b