Rollup merge of rust-lang#146535 - joshtriplett:mbe-unsafe-attr, r=petrochenkov

mbe: Implement `unsafe` attribute rules

This implements `unsafe attr` rules for declarative `macro_rules!` attributes, as specified in [RFC 3697](rust-lang/rfcs#3697).

An invocation of an attribute that uses an `unsafe attr` rule requires the `unsafe(attr(...))` syntax.

An invocation of an attribute that uses an ordinary `attr` rule must *not* use the `unsafe(attr(...))` syntax.

`unsafe` is only supported on an `attr` rule, not any other kind of `macro_rules!` rule.

Tracking issue for `macro_rules!` attributes: rust-lang#143547

Author: matthiaskrgr