Rollup merge of #144579 - joshtriplett:mbe-attr, r=petrochenkov

Implement declarative (`macro_rules!`) attribute macros (RFC 3697)

This implements [RFC 3697](https://github.com/rust-lang/rust/issues/143547), "Declarative (`macro_rules!`) attribute macros".

I would suggest reading this commit-by-commit. This first introduces the
feature gate, then adds parsing for attribute rules (doing nothing with them),
then adds the ability to look up and apply `macro_rules!` attributes by path,
then adds support for local attributes, then adds a test, and finally makes
various improvements to errors.

compiler/rustc_errors/src/emitter.rs

@@ -409,7 +409,7 @@ pub trait Emitter {
                 if !redundant_span || always_backtrace {
                     let msg: Cow<'static, _> = match trace.kind {
                         ExpnKind::Macro(MacroKind::Attr, _) => {
-                            "this procedural macro expansion".into()
+                            "this attribute macro expansion".into()
                         }
                         ExpnKind::Macro(MacroKind::Derive, _) => {
                             "this derive macro expansion".into()