fix(rustc_parse): incorrect span information for macro path expr

Old error output:

    3  |         let _: usize = $f;
       |                -----     ^ expected `usize`, found struct `Baz`
       |                |
       |                expected due to this

New error output:

    3  |         let _: usize = $f;
       |                -----   ^^ expected `usize`, found struct `Baz`
       |                |
       |                expected due to this

compiler/rustc_parse/src/parser/expr.rs

@@ -41,7 +41,7 @@ macro_rules! maybe_whole_expr {
                     let path = path.clone();
                     $p.bump();
                     return Ok($p.mk_expr(
-                        $p.token.span,
+                        $p.prev_token.span,
                         ExprKind::Path(None, path),
                         AttrVec::new(),
                     ));