libsyntax: use char::is_whitespace instead of custom implementations

Fixes #29590.

src/test/run-pass/parser-unicode-whitespace.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+// Beware editing: it has numerous whitespace characters which are important
+pub fn main() {
+    assert_eq!(4 + 　7 * 2
+
+
+/ 3 * 2 , 4 + 7 * 2 / 3 * 2);
+}