Add C++17 deduction guide for std::basic_string (P0433R2, partial)

Add C++17 deduction guide for std::basic_string (P0433R2, partial)

	* include/bits/alloc_traits.h (__is_allocator, _RequireAllocator):
	New trait and alias for detecting Allocator-like types.
	* include/bits/basic_string.h (basic_string): Add deduction guide
	from P0433.
	* include/ext/alloc_traits.h (__gnu_cxx::__alloc_traits): Add template
	parameter with default template argument that causes substitution
	failures for types that cannot be allocators.
	* testsuite/21_strings/basic_string/cons/char/deduction.cc: New.
	* testsuite/21_strings/basic_string/cons/wchar_t/deduction.cc: New.

From-SVN: r248967