Clang: Add commentary about column convertion

... to/from utf8 byte offset used by Clang.

Change-Id: I294d6cd61b416e5f2d64206ee2f3f1b4a91fb1d3
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/clangcodemodel/clangutils.cpp

@@ -190,5 +190,15 @@ void setLastSentDocumentRevision(const QString &filePath, uint revision)
         document->sendTracker().setLastSentRevision(int(revision));
 }
 
+int clangColumn(const QString &lineText, int cppEditorColumn)
+{
+    // (1) cppEditorColumn is the actual column shown by CppEditor.
+    // (2) The return value is the column in Clang which is the utf8 byte offset from the beginning
+    //     of the line.
+    // Here we convert column from (1) to (2).
+    // '+ 1' is for 1-based columns
+    return lineText.left(cppEditorColumn).toUtf8().size() + 1;
+}
+
 } // namespace Utils
 } // namespace Clang