Fix msg for verbose suggestions with confusable capitalization

When encountering a verbose/multipart suggestion that has changes
that are only caused by different capitalization of ASCII letters that have
little differenciation, expand the message to highlight that fact (like we
already do for inline suggestions).

The logic to do this was already present, but implemented incorrectly.

compiler/rustc_errors/src/lib.rs

@@ -319,7 +319,9 @@ impl CodeSuggestion {
                 // We need to keep track of the difference between the existing code and the added
                 // or deleted code in order to point at the correct column *after* substitution.
                 let mut acc = 0;
+                let mut only_capitalization = false;
                 for part in &substitution.parts {
+                    only_capitalization |= is_case_difference(sm, &part.snippet, part.span);
                     let cur_lo = sm.lookup_char_pos(part.span.lo());
                     if prev_hi.line == cur_lo.line {
                         let mut count =
@@ -392,7 +394,6 @@ impl CodeSuggestion {
                     }
                 }
                 highlights.push(std::mem::take(&mut line_highlight));
-                let only_capitalization = is_case_difference(sm, &buf, bounding_span);
                 // if the replacement already ends with a newline, don't print the next line
                 if !buf.ends_with('\n') {
                     push_trailing(&mut buf, prev_line.as_ref(), &prev_hi, None);