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

Old error output:

   = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
help: wrap this expression in parentheses
   |
4  |             break '_l $f(;)
   |                         ^ ^

New error output:

   = note: this warning originates in the macro `foo` (in Nightly builds, run with -Z macro-backtrace for more info)
help: wrap this expression in parentheses
   |
4  |             break '_l ($f);
   |                       ^  ^

compiler/rustc_parse/src/parser/expr.rs

@@ -50,7 +50,7 @@ macro_rules! maybe_whole_expr {
                     let block = block.clone();
                     $p.bump();
                     return Ok($p.mk_expr(
-                        $p.token.span,
+                        $p.prev_token.span,
                         ExprKind::Block(block, None),
                         AttrVec::new(),
                     ));