Rollup merge of #50610 - estebank:fmt-str, r=Kimundi

Improve format string errors

Point at format string position inside the formatting string:
```
error: invalid format string: unmatched `}` found
  --> $DIR/format-string-error.rs:21:22
   |
LL |     let _ = format!("}");
   |                      ^ unmatched `}` in format string
```

Explain that argument names can't start with an underscore:
```
error: invalid format string: invalid argument name `_foo`
  --> $DIR/format-string-error.rs:15:23
   |
LL |     let _ = format!("{_foo}", _foo = 6usize);
   |                       ^^^^ invalid argument name in format string
   |
   = note: argument names cannot start with an underscore
```

Fix #23476.

The more accurate spans will only be seen when using `format!` directly, when using `println!` the diagnostics machinery makes the span be the entire statement.

src/libsyntax_pos/lib.rs

@@ -428,6 +428,13 @@ impl Span {
         )
     }
 
+    pub fn from_inner_byte_pos(self, start: usize, end: usize) -> Span {
+        let span = self.data();
+        Span::new(span.lo + BytePos::from_usize(start),
+                  span.lo + BytePos::from_usize(end),
+                  span.ctxt)
+    }
+
     #[inline]
     pub fn apply_mark(self, mark: Mark) -> Span {
         let span = self.data();