CppEditor: Fix highlighting angle brackets

The code assumed that all highlighting results come in at once, which is
no longer true as of d6f5d07639.

Change-Id: I5ed6baf88956d64a30ee3fb236d4e2575a7f80c9
Reviewed-by: David Schulz <david.schulz@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/texteditor/textdocumentlayout.h

@@ -17,8 +17,9 @@
 
 namespace TextEditor {
 
-struct TEXTEDITOR_EXPORT Parenthesis
+class TEXTEDITOR_EXPORT Parenthesis
 {
+public:
     enum Type : char { Opened, Closed };
 
     Parenthesis() = default;