Improve "Go to Definition" for code included with include! (e.g. bindgen bindings)

[jschwe]

Currently "Go to definition" on a symbol / function generated by bindgen brings you to the include!(...) macro, where the bindings were included. I think it would be a great addition if rust-analyzer could also support directly opening the bindings rust file and showing the symbol there.
Typically the bindings.rs file would be in OUT_DIR, e.g. for include!(concat!(env!("OUT_DIR"), "/bindings.rs")), but since rust_analyzer already seems to be parsing the bindings.rs file to some extent, maybe this is simple to implement?

[flodiebold]

Duplicate of #3767.