Auto merge of #37246 - goffrie:no-loop, r=jseyfried

Don't spin expanding stmt macros.

If we can't make progress when parsing a macro expansion as a statement then we should just bail.

This alleviates the symptoms shown in e.g. #37113 and #37234 but it doesn't fix the problem that parsing invalid enum bodies (and others) leaves the parser in a crappy state.

I'm not sold on this strategy (checking `tokens_consumed`), so if anyone has a better idea, I'm all ears!

src/test/parse-fail/issue-37234.rs

@@ -0,0 +1,19 @@
+// Copyright 2016 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.
+
+macro_rules! failed {
+    () => {{
+        let x = 5 ""; //~ ERROR found `""`
+    }} //~ ERROR macro expansion ignores token `}`
+}
+
+fn main() {
+    failed!();
+}