ClangFormat: Add context menu item to open current config file

Add the context menu item to C++ editor which allow you to open
the currently used .clang-format configuration file.

Change-Id: If9d6bec4f4085c18b13df15d78417aee82ddb4e4
Reviewed-by: Marco Bubke <marco.bubke@qt.io>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>

src/plugins/clangformat/clangformatutils.h

@@ -44,6 +44,7 @@ clang::format::FormatStyle currentProjectStyle();
 clang::format::FormatStyle currentGlobalStyle();
 
 // Is the style from the matching .clang-format file or global one if it's not found.
+QString configForFile(Utils::FileName fileName);
 clang::format::FormatStyle styleForFile(Utils::FileName fileName);
 
 }