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

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

This implements [RFC 3697](rust-lang/rust#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.

Author: tgross35

tests/fail/alloc/alloc_error_handler_custom.stderr

@@ -11,7 +11,7 @@ note: inside `_::__rg_oom`
   --> tests/fail/alloc/alloc_error_handler_custom.rs:LL:CC
    |
 LL | #[alloc_error_handler]
-   | ---------------------- in this procedural macro expansion
+   | ---------------------- in this attribute macro expansion
 LL | fn alloc_error_handler(layout: Layout) -> ! {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: inside `alloc::alloc::handle_alloc_error::rt_error` at RUSTLIB/alloc/src/alloc.rs:LL:CC