libstdc++: Improve doxygen docs for <regex>

libstdc++: Improve doxygen docs for <regex>

Add @headerfile and @since tags. Improve grouping of non-member
functions via @relates tags.

Mark the std::pair base class of std::sub_match as undocumented, so that
the docs don't show all the related non-member functions are part of the
sub_match API. Use a new macro to re-add the data members for Doxygen
only.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Define macro
	_GLIBCXX_DOXYGEN_ONLY to expand its argument.
	* include/bits/c++config (_GLIBCXX_DOXYGEN_ONLY): Define.
	* include/bits/regex.h: Improve doxygen docs.
	* include/bits/regex_constants.h: Likewise.
	* include/bits/regex_error.h: Likewise.