8122: Make bare underscore token an Ident rather than Punct in proc-macro r=edwin0cheng a=kevinmehall
In rustc and proc-macro2, a bare `_` token is parsed for procedural macro purposes as `Ident` rather than `Punct` (see https://github.com/rust-lang/rust/pull/48842). This changes rust-analyzer to match rustc's behavior and implementation by handling `_` as an Ident in token trees, but explicitly preventing `$x:ident` from matching it in MBE.
proc macro crate:
```rust
#[proc_macro]
pub fn input(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
dbg!(input)
}
```
test crate:
```rust
test_proc_macro::input!(_);
```
output (rustc):
```rust
[test-proc-macro/src/lib.rs:10] input = TokenStream [
Ident {
ident: "_",
span: #0 bytes(173..174),
},
]
```
output (rust-analyzer before this change):
```rust
[test-proc-macro/src/lib.rs:10] input = TokenStream [
Punct {
ch: '_',
spacing: Joint,
span: 4294967295,
},
]
```
output (rust-analyzer after this change):
```rust
[test-proc-macro/src/lib.rs:10] input = TokenStream [
Ident {
ident: "_",
span: 4294967295,
},
]
```
Co-authored-by: Kevin Mehall <km@kevinmehall.net>
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