P0935R0 Eradicating unnecessarily explicit default constructors

P0935R0 Eradicating unnecessarily explicit default constructors

This is the last remaining piece of P0935R0. This adds a default
constructor to each of the streambuf and stream types in <sstream> so
that default construction does not use the 'explicit' constructor that
has a single, defaulted argument.

	P0935R0 Eradicating unnecessarily explicit default constructors
	* config/abi/pre/gnu.ver: Tighten existing patterns and export new
	default constructor symbols.
	* include/std/sstream (basic_stringbuf, basic_istringstream)
	(basic_ostringstream, basic_stringstream): Remove default arguments
	from explicit constructors taking ios_base::openmode and add separate
	non-explicit default constructors.
	* testsuite/27_io/basic_istringstream/cons/default.cc: New.
	* testsuite/27_io/basic_ostringstream/cons/default.cc: New.
	* testsuite/27_io/basic_stringstream/cons/default.cc: New.
	* testsuite/27_io/basic_stringbuf/cons/char/default.cc: New.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/default.cc: New.

From-SVN: r262474