Clang: Refactor icon setting in ClangTextMark

Change-Id: Ie0c2185d83d3b8344c2f88467236c1eac3b07a6b
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>

src/plugins/clangcodemodel/clangtextmark.h

@@ -46,6 +46,9 @@ public:
 
     ClangTextMark(ClangTextMark &other) = delete;
     ClangTextMark &operator=(ClangTextMark &other) = delete;
+
+private:
+    void setIcon(ClangBackEnd::DiagnosticSeverity severity);
 };
 
 } // namespace ClangCodeModel