Implement LWG 2758.

Implement LWG 2758.

	* include/bits/basic_string.h
	(append(__sv_type, size_type, size_type)): Turn into a template,
	change parameter type, constrain, add a conversion to __sv_type
	from the dependent parameter type.
	(assign(__sv_type, size_type, size_type)): Likewise.
	(insert(size_type, __sv_type, size_type, size_type)): Likewise.
	(replace(size_type, size_type, __sv_type, size_type, size_type)):
	Likewise.
	(compare(size_type, size_type,__sv_type, size_type, size_type)):
	Likewise.
	* testsuite/21_strings/basic_string/lwg2758.cc: New.

From-SVN: r239370