Rollup merge of rust-lang#141996 - Daniel-Aaron-Bloom:dollar_crate, r=petrochenkov

Fix `proc_macro::Ident`'s handling of `$crate`

This PR is addresses a few minor bugs, all relating to `proc_macro::Ident`'s support for `$crate`. `Ident` currently supports `$crate` (as can be seen in the `mixed-site-span` test), but:
* `proc_macro::Symbol::can_be_raw` is out of sync with `rustc_span::Symbol::can_be_raw`
  * former doesn't cover `$crate` while the latter does cover `kw::DollarCrate`
* `Ident::new` rejects `$crate`
  * This conflicts with the [reference definition](https://doc.rust-lang.org/nightly/reference/macros-by-example.html#r-macro.decl.meta.specifier) of `ident` which includes `$crate`.
  * This also conflicts with the documentation on [`Display for Ident`](https://doc.rust-lang.org/proc_macro/struct.Ident.html#impl-Display-for-Ident) which says the output "should be losslessly convertible back into the same identifier".

This PR fixes the above issues and extends the `mixed-site-span` test to exercise these fixed code paths, as well as validating the different possible spans resolve `$crate`  as expected (for both the new and old `$crate` construction code paths).

Author: tgross35

proc_macro/src/bridge/symbol.rs

@@ -33,7 +33,7 @@ impl Symbol {
     /// Validates and normalizes before converting it to a symbol.
     pub(crate) fn new_ident(string: &str, is_raw: bool) -> Self {
         // Fast-path: check if this is a valid ASCII identifier
-        if Self::is_valid_ascii_ident(string.as_bytes()) {
+        if Self::is_valid_ascii_ident(string.as_bytes()) || string == "$crate" {
             if is_raw && !Self::can_be_raw(string) {
                 panic!("`{}` cannot be a raw identifier", string);
             }
@@ -79,7 +79,7 @@ impl Symbol {
     // Mimics the behavior of `Symbol::can_be_raw` from `rustc_span`
     fn can_be_raw(string: &str) -> bool {
         match string {
-            "_" | "super" | "self" | "Self" | "crate" => false,
+            "_" | "super" | "self" | "Self" | "crate" | "$crate" => false,
             _ => true,
         }
     }