More pieces of P0935R0, making default constructors non-explicit.

	* include/backward/strstream (strstreambuf): Add non-explicit default
	constructor.
	* include/bits/locale_conv.h (wbuffer_convert, wstring_convert):
	Likewise.
	* include/bits/regex.h (match_results): Likewise.
	* testsuite/22_locale/conversions/buffer/1.cc: Test for non-explicit
	default constructor.
	* testsuite/22_locale/conversions/string/1.cc: Likewise.
	* testsuite/28_regex/match_results/ctors/char/default.cc: Likewise.
	* testsuite/28_regex/match_results/ctors/wchar_t/default.cc: Likewise.

From-SVN: r261597