Make unicode text flow control chars visible as �

We already point these out quite aggressively, telling people not to use them, but would normally be rendered as nothing. Having them visible will make it easier for people to actually deal with them.

```
error: unicode codepoint changing visible direction of text present in literal
  --> $DIR/unicode-control-codepoints.rs:26:22
   |
LL |     println!("{:?}", '�');
   |                      ^-^
   |                      ||
   |                      |'\u{202e}'
   |                      this literal contains an invisible unicode text flow control codepoint
   |
   = note: these kind of unicode codepoints change the way text flows on applications that support them, but can cause confusion because they change the order of characters on the screen
   = help: if their presence wasn't intentional, you can remove them
help: if you want to keep them but make them visible in your source code, you can escape them
   |
LL |     println!("{:?}", '\u{202e}');
   |                       ~~~~~~~~
```

vs the previous

```
error: unicode codepoint changing visible direction of text present in literal
  --> $DIR/unicode-control-codepoints.rs:26:22
   |
LL |     println!("{:?}", '');
   |                      ^-
   |                      ||
   |                      |'\u{202e}'
   |                      this literal contains an invisible unicode text flow control codepoint
   |
   = note: these kind of unicode codepoints change the way text flows on applications that support them, but can cause confusion because they change the order of characters on the screen
   = help: if their presence wasn't intentional, you can remove them
help: if you want to keep them but make them visible in your source code, you can escape them
   |
LL |     println!("{:?}", '\u{202e}');
   |                       ~~~~~~~~
```

1 files changed, 2 insertions, 1 deletions

diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index ea57c1ce818..7c8ac3be4be 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -2093,7 +2093,8 @@ pub fn char_width(ch: char) -> usize {
         | '\u{000E}' | '\u{000F}' | '\u{0010}' | '\u{0011}' | '\u{0012}' | '\u{0013}'
         | '\u{0014}' | '\u{0015}' | '\u{0016}' | '\u{0017}' | '\u{0018}' | '\u{0019}'
         | '\u{001A}' | '\u{001B}' | '\u{001C}' | '\u{001D}' | '\u{001E}' | '\u{001F}'
-        | '\u{007F}' => 1,
+        | '\u{007F}' | '\u{202A}' | '\u{202B}' | '\u{202D}' | '\u{202E}' | '\u{2066}'
+        | '\u{2067}' | '\u{2068}' | '\u{202C}' | '\u{2069}' => 1,
         _ => unicode_width::UnicodeWidthChar::width(ch).unwrap_or(1),
     }
 }