format macro argument parsing fix

When the character next to `{}` is "shifted" (when mapping a byte index
in the format string to span) we should avoid shifting the span end
index, so first map the index of `}` to span, then bump the span,
instead of first mapping the next byte index to a span (which causes
bumping the end span too much).

Regression test added.

Fixes #83344

tests/ui/write_literal_2.stderr

@@ -75,8 +75,9 @@ LL |         "1", "2", "3",
    |
 help: try this
    |
-LL |         "some 1{} / {}", "2", "3",
-   |               ^        --
+LL |         "some 1/
+LL |         {} / {}", "2", "3",
+   |
 
 error: literal with an empty format string
   --> $DIR/write_literal_2.rs:25:14