Add std::string constructor for substring of string_view (LWG 2742)

Add std::string constructor for substring of string_view (LWG 2742)

	* doc/xml/manual/intro.xml: Document LWG 2742 status.
	* doc/html/*: Regenerate.
	* include/bits/basic_string.h
	(basic_string(const T&, size_type, size_type, const Allocator&)): Add
	constructor for substring of basic_string_view, as per LWG 2742 but
	with additional constraint to fix ambiguity.
	* testsuite/21_strings/basic_string/cons/char/9.cc: New test.
	* testsuite/21_strings/basic_string/cons/wchar_t/9.cc: New test.

From-SVN: r242416