Clang: Add -Wno-used-but-marked-unused

...to built-in diagnostic configuration "Warnings for almost
everything".

Change-Id: I2a70c76c5b876cc0dc8cebd80c2457550b608593
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/cpptools/clangdiagnosticconfigsmodel.cpp

@@ -84,6 +84,7 @@ static void addConfigForAlmostEveryWarning(ClangDiagnosticConfigsModel &model)
         QStringLiteral("-Wno-documentation"),
         QStringLiteral("-Wno-shadow"),
         QStringLiteral("-Wno-missing-prototypes"), // Not optimal for C projects.
+        QStringLiteral("-Wno-used-but-marked-unused"), // e.g. QTest::qWait
     } + commonOptions());
 
     model.appendOrUpdate(config);