Remove Path::is_ident.

It checks that a path has a single segment that matches the given
symbol, and that there are zero generic arguments. It has a single use.

We also have `impl PartialEq<Symbol> for Path` which does exactly the
same thing *except* it doesn't check for zero generic arguments, which
seems like an oversight. It has numerous uses.

This commit removes `Path::is_ident`, adds a test for zero generic
arguments to `PartialEq<Symbol> for Path`, and changes the single use of
`is_ident` to instead use `==`.

compiler/rustc_parse/src/parser/expr.rs

@@ -834,7 +834,7 @@ impl<'a> Parser<'a> {
         // guides recovery in case we write `&raw expr`.
         if borrow_kind == ast::BorrowKind::Ref
             && mutbl == ast::Mutability::Not
-            && matches!(&expr.kind, ExprKind::Path(None, p) if p.is_ident(kw::Raw))
+            && matches!(&expr.kind, ExprKind::Path(None, p) if *p == kw::Raw)
         {
             self.expected_token_types.insert(TokenType::KwMut);
             self.expected_token_types.insert(TokenType::KwConst);