`windows-rdl` overload attribute should be supported directly

The WinRT metadata supports the `Overload` attribute that provides the non-overload name of an interface method. This has always been logically backward. While we cannot change the metadata, we can have RDL use the actual method names and use a special `#[overload(NAME)]` attribute that provides the name of the overload.

Start by locating the windows-rdl metadata handling for the WinRT Overload attribute and the logic that assigns interface method names. Confirm how RDL represents attributes, then make it support actual method names plus #[overload(NAME)]; done means the special attribute supplies the overload name without changing the metadata.

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