Auto merge of #41174 - estebank:issue-41155, r=nikomatsakis

Point at only one char on `Span::next_point`

Avoid pointing at two chars so the diagnostic output doesn't display a
multiline span when starting beyond a line end.

Fix #41155.

Instead of

```rust
error: expected one of `(`, `const`, `default`, `extern`, `fn`, `type`, or `unsafe`, found `}`
 --> <anon>:3:1
  |
1 |   impl S {	pub
  |  _____________- starting here...
2 | |
  | |  ...ending here: expected one of 7 possible tokens here
3 |   }
  |   ^ unexpected token
```

show

```rust
error: expected one of `(`, `const`, `default`, `extern`, `fn`, `type`, or `unsafe`, found `}`
  --> <anon>:13:1
   |
12 |     pub
   |        - expected one of 7 possible tokens here
13 | }
   | ^ unexpected token
```

src/test/ui/token/issue-41155.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 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.
+
+impl S {
+    pub
+}

src/test/ui/token/issue-41155.stderr

@@ -0,0 +1,10 @@
+error: expected one of `(`, `const`, `default`, `extern`, `fn`, `type`, or `unsafe`, found `}`
+  --> $DIR/issue-41155.rs:13:1
+   |
+12 |     pub
+   |        - expected one of 7 possible tokens here
+13 | }
+   | ^ unexpected token
+
+error: aborting due to previous error
+