libstdc++: Fix new <sstream> constructors

libstdc++: Fix new <sstream> constructors

- Add a missing 'explicit' to a basic_stringbuf constructor.
- Set up the get/put area pointers in the constructor from strings using
  different allocator types.
- Remove public basic_stringbuf::__sv_type alias.
- Do not construct temporary basic_string objects with a
  default-constructed allocator.

Also, change which basic_string constructor is used, as a minor
compile-time optimization. Constructing from a basic_string_view
requires more work from the compiler, so just use a pointer and length.

libstdc++-v3/ChangeLog:

	* include/std/sstream (basic_stringbuf(const allocator_type&):
	Add explicit.
	(basic_stringbuf(const basic_string<C,T,SA>&, openmode, const A&)):
	Call _M_stringbuf_init. Construct _M_string from pointer and length
	to avoid constraint checks for string view.
	(basic_stringbuf::view()): Make __sv_type alias local to the
	function.
	(basic_istringstream(const basic_string<C,T,SA>&, openmode, const A&)):
	Pass string to _M_streambuf instead of constructing a temporary
	with the wrong allocator.
	(basic_ostringstream(const basic_string<C,T,SA>&, openmode, const A&)):
	Likewise.
	(basic_stringstream(const basic_string<C,T,SA>&, openmode, const A&)):
	Likewise.
	* src/c++20/sstream-inst.cc: Use string_view and wstring_view
	typedefs in explicit instantiations.
	* testsuite/27_io/basic_istringstream/cons/char/1.cc: Add more
	tests for constructors.
	* testsuite/27_io/basic_ostringstream/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/cons/char/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/cons/char/2.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/1.cc: Likewise.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/2.cc: Likewise.
	* testsuite/27_io/basic_stringstream/cons/char/1.cc: Likewise.