Auto merge of #111780 - weiznich:diagnostic_namespace, r=petrochenkov

Diagnostic namespace

This PR implements the basic infrastructure for accepting the `#[diagnostic]` attribute tool namespace as specified in rust-lang/rfcs#3368. Note: This RFC is not merged yet, but it seems like it will be accepted soon. I open this PR early on to get feedback on the actual implementation as soon as possible. This hopefully enables getting at least the diagnostic namespace to stable rust "soon", so that crates do not need to bump their MSRV if we stabilize actual attributes in this namespace.

 This PR only adds infrastructure accept attributes from this namespace, it does not add any specific attribute. Therefore the compiler will emit a lint warning for each attribute that's actually used. This namespace is added behind a feature flag, so it will be only available on a nightly compiler for now.

cc `@estebank` as they've supported me in planing, specifying and implementing this feature.