PR libstdc++/79162 implement LWG 2946 and LWG 2758

PR libstdc++/79162 implement LWG 2946 and LWG 2758

2017-09-04  Daniel Kruegler  <daniel.kruegler@gmail.com>

	PR libstdc++/79162
	Implement LWG 2946, LWG 2758's resolution missed further corrections
	* include/bits/basic_string.h (basic_string::compare): Add missing
	required noexcept specifications.
	(basic_string): Introduce internal _S_to_string_view and __sv_wrapper
	for implicit string_view conversion.
	(basic_string::basic_string): Fix explicit string_view conversion by
	implicit conversion using _S_to_string_view and __sv_wrapper.
	(basic_string): Introduce internal basic_string(__sv_wrapper, Alloc)
	constructor.
	(basic_string): Fix operator=(T) template by operator=(const T&)
	template for uncopyable types (PR 79162).
	(basic_string::operator+=, basic_string::append, basic_string::assign)
	(basic_string::insert, basic_string::replace, basic_string::find)
	(basic_string::rfind, basic_string::find_first_of)
	(basic_string::find_last_of, basic_string::find_first_not_of)
	(basic_string::find_last_not_of, basic_string::compare): Replace
	__sv_type argument by template const T& (LWG 2946) and correct
	documentation describing __sv_type argument.
	(basic_string::find, basic_string::rfind, basic_string::find_first_of)
	(basic_string::find_last_of, basic_string::find_first_not_of)
	(basic_string::find_last_not_of, basic_string::compare): Replace
	unconditional noexcept specification by conditional noexcept
	specification to partially balance the removal of noexcept by LWG 2946.
	* testsuite/21_strings/basic_string/79162.cc: New.
	* testsuite/21_strings/basic_string/lwg2946.cc: New.

From-SVN: r251664