GCC suggest-override warnings

Complete list of warnings when Boost 1.72 is built with GCC 7.5 with `-Wsuggest-override` added to `cxxflags`. Duplicated warnings from same location are omitted:

./boost/regex/v4/cpp_regex_traits.hpp:102:1: warning: ‘std::basic_streambuf<_CharT, _Traits>* boost::re_detail_107200::parser_buf::setbuf(boost::re_detail_107200::parser_buf::char_type*, boost::re_detail_107200::parser_buf::streamsize) [with charT = char; traits = std::char_traits; boost::re_detail_107200::parser_buf::char_type = char; boost::re_detail_107200::parser_buf::streamsize = long int]’ can be marked override [-Wsuggest-override]

./boost/regex/v4/cpp_regex_traits.hpp:110:1: warning: ‘boost::re_detail_107200::parser_buf::pos_type boost::re_detail_107200::parser_buf::seekoff(boost::re_detail_107200::parser_buf::off_type, std::ios_base::seekdir, std::ios_base::openmode) [with charT = char; traits = std::char_traits; boost::re_detail_107200::parser_buf::pos_type = std::fpos<__mbstate_t>; boost::re_detail_107200::parser_buf::off_type = long int; std::ios_base::seekdir = std::_Ios_Seekdir; std::ios_base::openmode = std::_Ios_Openmode]’ can be marked override [-Wsuggest-override]

./boost/regex/v4/cpp_regex_traits.hpp:156:1: warning: ‘boost::re_detail_107200::parser_buf::pos_type boost::re_detail_107200::parser_buf::seekpos(boost::re_detail_107200::parser_buf::pos_type, std::ios_base::openmode) [with charT = char; traits = std::char_traits; boost::re_detail_107200::parser_buf::pos_type = std::fpos<__mbstate_t>; std::ios_base::openmode = std::_Ios_Openmode]’ can be marked override [-Wsuggest-override]

`BOOST_OVERRIDE` was introduced in https://github.com/boostorg/config/commit/ffe4e0f5a448578cce14e3eed0cf7163333a81d9.

Start in boost/regex/v4/cpp_regex_traits.hpp at lines 102, 110, and 156, and inspect the surrounding parser_buf methods and the referenced BOOST_OVERRIDE addition. Rebuild Boost 1.72 with GCC 7.5 and -Wsuggest-override to confirm the listed warnings; done means those warnings no longer appear.

Written by the indexing model from the issue text.