re PR libstdc++/61374 (string_view::operator string() is buggy)

re PR libstdc++/61374 (string_view::operator string() is buggy)

	PR libstdc++/61374
	* include/experimental/string_view (operator basic_string): Correct
	order of arguments.
	(to_string): Replace with member function.
	Add inline specifiers. Remove unused header. Remove _S_empty_rep and
	allow _M_str to be null.
	* testsuite/experimental/string_view/cons/char/1.cc: Adjust to new
	default constructor semantics.
	* testsuite/experimental/string_view/cons/wchar_t/1.cc: Likewise.
	* testsuite/experimental/string_view/operations/copy/char/1.cc: Fix
	copyright dates. Remove unused header.
	* testsuite/experimental/string_view/operations/copy/wchar_t/1.cc:
	Likewise.
	* testsuite/experimental/string_view/operations/data/char/1.cc:
	Fix copyright dates. Adjust to new default constructor semantics.
	* testsuite/experimental/string_view/operations/data/wchar_t/1.cc:
	Likewise.
	* testsuite/experimental/string_view/operations/to_string/1.cc: New.

From-SVN: r211113