Auto merge of #40346 - jseyfried:path_and_tokenstream_attr, r=nrc

`TokenStream`-based attributes, paths in attribute and derive macro invocations

This PR
 - refactors `Attribute` to use  `Path` and `TokenStream` instead of `MetaItem`.
 - supports macro invocation paths for attribute procedural macros.
   - e.g. `#[::foo::attr_macro] struct S;`, `#[cfg_attr(all(), foo::attr_macro)] struct S;`
 - supports macro invocation paths for derive procedural macros.
   - e.g. `#[derive(foo::Bar, super::Baz)] struct S;`
 - supports arbitrary tokens as arguments to attribute procedural macros.
   - e.g. `#[foo::attr_macro arbitrary + tokens] struct S;`
 - supports using arbitrary tokens in "inert attributes" with derive procedural macros.
   - e.g. `#[derive(Foo)] struct S(#[inert arbitrary + tokens] i32);`
where `#[proc_macro_derive(Foo, attributes(inert))]`

r? @nrc

src/test/ui/span/E0536.stderr

@@ -2,7 +2,7 @@ error[E0536]: expected 1 cfg-pattern
   --> $DIR/E0536.rs:11:7
    |
 11 | #[cfg(not())] //~ ERROR E0536
-   |       ^^^^^
+   |       ^^^
 
 error: aborting due to previous error
 

src/test/ui/span/E0537.stderr

@@ -2,7 +2,7 @@ error[E0537]: invalid predicate `unknown`
   --> $DIR/E0537.rs:11:7
    |
 11 | #[cfg(unknown())] //~ ERROR E0537
-   |       ^^^^^^^^^
+   |       ^^^^^^^
 
 error: aborting due to previous error