Auto merge of #51496 - petrochenkov:mhelper2, r=nikomatsakis

Auto merge of #51496 - petrochenkov:mhelper2, r=nikomatsakis

Implement `#[macro_export(local_inner_macros)]` (a solution for the macro helper import problem)

Implement a solution for the macro helper issue discussed in https://github.com/rust-lang/rust/issues/35896 as described in https://github.com/rust-lang/rust/issues/35896#issuecomment-395977901.

Macros exported from libraries can be marked with `#[macro_export(local_inner_macros)]` and this annotation changes how nested macros in them are resolved.

If we have a fn-like macro call `ident!(...)` and `ident` comes from a `macro_rules!` macro marked with  `#[macro_export(local_inner_macros)]` then it's replaced with `$crate::ident!(...)` and resolved as such (`$crate` gets the same context as `ident`).