…=Veykril

Introduce macro sub-namespaces and `macro_use` prelude

This PR implements two mechanisms needed for correct macro name resolution: macro sub-namespace and `macro_use` prelude.

- [macro sub-namespaces][subns-ref]

  Macros have two sub-namespaces: one for function-like macro and the other for those in attributes (including custom derive macros). When we're resolving a macro name for function-like macro, we should ignore non-function-like macros, and vice versa.

  This helps resolve single-segment macro names because we can (and should, as rustc does) fallback to names in preludes when the name in the current module scope is in different sub-namespace.

- [`macro_use` prelude][prelude-ref]

  `#[macro_use]`'d extern crate declarations (including the standard library) bring their macros into scope, but they should not be prioritized over local macros (those defined in place and those explicitly imported).

  We have been bringing them into legacy (textual) macro scope, which has the highest precedence in name resolution. This PR introduces the `macro_use` prelude in crate-level `DefMap`s, whose precedence is lower than local macros but higher than the standard library prelude.

The first 3 commits are drive-by fixes/refactors.

Fixes rust-lang#8828 (prelude)
Fixes rust-lang#12505 (prelude)
Fixes rust-lang#12734 (prelude)
Fixes rust-lang#13683 (prelude)
Fixes rust-lang#13821 (prelude)
Fixes rust-lang#13974 (prelude)
Fixes rust-lang#14254 (namespace)

[subns-ref]: https://doc.rust-lang.org/reference/names/namespaces.html#sub-namespaces
[prelude-ref]: https://doc.rust-lang.org/reference/names/preludes.html#macro_use-prelude

Closes rust-lang/rust-clippy#13169

Late lints now use a parent iter to check for `#[clippy::msrv]`
attributes instead of keeping track with `extract_msrv_attr`. This is
required for incremental lints since they run per module instead of per
crate so don't visit all the necessary attributes

As a basic optimisation if no `#[clippy::msrv]` attributes are
discovered in early passes the HIR access is skipped completely and just
the configured MSRV is used, for most code bases this will be the case

changelog: none