"Go To Definition" on a macro-generated type should go to the macro output, not its invocation

[SludgePhD]

Currently, "go to definition" on a type that was generated by a macro takes you to the macro invocation. This is extremely inconvenient for complicated macros that generate a lot of code. It would be far more convenient if it took you to the expansion of the macro instead.

rust-analyzer is already capable of showing the macro expansion with the "expand macro recursively" command, and it knows which specific macro invocation a definition comes from, so it should already have all the information needed to take you there directly.

[flodiebold]

The macro expansion isn't really a location we can go to, so I'm not sure how possible this is. There are some related/alternative ideas in #7890.

[Veykril]

We could probably make it work with an lsp extension to allow the server to allocate "fake files" on the client side. Anything but trivial though for a variety of reasons.