Export explicit instantiations for C++17 members of std::string

Export explicit instantiations for C++17 members of std::string

The C++17 standard added some new members to std::basic_string, which
were not previously instantiated in the library. This meant that the
extern template declarations had to be disabled for C++17 mode. With
this patch the new members are instantiated in the library and so the
explicit instantiation declarations can be used for C++17.

The new members added by C++2a are still not exported, and so the
explicit instantiation declarations are still disabled for C++2a.

	* config/abi/pre/gnu.ver (GLIBCXX_3.4.21): Make patterns less greedy
	for const member functions of std::basic_string.
	(GLIBCXX_3.4.26): Export member functions of std::basic_string added
	in C++17.
	* include/bits/basic_string.h (basic_string(__sv_wrapper, const A&)):
	Make non-standard constructor private.
	[!_GLIBCXX_USE_CXX11_ABI] (basic_string(__sv_wrapper, const A&)):
	Likewise.
	* include/bits/basic_string.tcc (std::string, std::wstring): Declare
	explicit instantiations for C++17 as well as earlier dialects.
	* src/c++17/Makefile.am: Add new source files.
	* src/c++17/Makefile.in: Regenerate.
	* src/c++17/cow-string-inst.cc: New file defining explicit
	instantiations for basic_string member functions added in C++17.
	* src/c++17/string-inst.cc: Likewise.

From-SVN: r267585