Fix Clang and Clang-tidy warnings in Qt headers

Please fix next Clang and Clang-tidy (version 9) warnings in Qt headers:

<pre>
include/QtCore/qlist.h:632:58: warning: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Wextra-semi-stmt]
include/QtCore/qlist.h:666:59: warning: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Wextra-semi-stmt]
include/QtCore/qlist.h:700:59: warning: empty expression statement has no effect; remove unnecessary ';' to silence this warning [-Wextra-semi-stmt]
include/QtWidgets/qstylepainter.h:63:6: warning: extra ';' after member function definition [-Wextra-semi]
</pre>

Destructors in derived classes are not marked with `override`. Warnings appear in other components too. For example:

<pre>
include/QtCore/qbuffer.h:66:5: warning: '~QBuffer' overrides a destructor but is not marked 'override' [-Winconsistent-missing-destructor-override]
include/QtCore/qbuffer.h:66:5: warning: annotate this function with 'override' or (rarely) 'final' [modernize-use-override]
</pre>

There are also a lot of -Wused-but-marked-unused warnings on Ubuntu 18.04. For example:

<pre>
include/QtCore/qsize.h:303:10: warning: 'qIsNull' was marked unused but was used [-Wused-but-marked-unused]
</pre>