Fix basic_string_view typedefs and enforce preconditions

Fix basic_string_view typedefs and enforce preconditions

The basic_string_view::pointer and basic_string_view::reference typedefs
are supposed to refer to the non-const value type.

In previous standards having traits_type::char_type different to
value_type was simply undefined, but in the C++2a draft it's ill-formed,
as changed by P1148R0. For std::basic_string and iostreams we might
want to only enforce this conditionally for __cplusplus > 201703L but
for std::basic_string_view we don't have backwards compatibility
concerns. Also add assertions to verify the _CharT argument is a
"char-like" type (non-array, trivial, standard layout type).

Also remove the non-standard basic_string_view::_M_check and
basic_string_view::_M_limit member functions, replacing them with
non-member functions that will still exist even if basic_string_view is
specialized by the program.

	* include/experimental/string_view (basic_string_view::pointer)
	(basic_string_view::reference): Fix to refer to non-const value_type.
	* include/bits/basic_string.h (basic_string): Use __sv_check and
	__sv_limit instead of basic_string_view::_M_check and
	basic_string_view::_M_limit.
	* include/std/string_view (__sv_check, __sv_limit): New
	helper functions to replace basic_string_view::_M_check and
	basic_string_view::_M_limit.
	(basic_string_view): Add static assertions to enforce ill-formed
	requirement for traits_type::char_type from P1148R0, and to enforce
	required properties of char-like types.
	(basic_string_view::pointer, basic_string_view::reference): Fix to
	refer to non-const value_type.
	(basic_string_view::operator[], basic_string_view::at)
	(basic_string_view::front, basic_string_view::back)
	(basic_string_view::data): Use const_reference and const_pointer
	typedefs for return types.
	(basic_string_view::_M_check, basic_string_view::_M_limit): Remove.
	(hash<wstring_view>): Fix argument_type typedef.
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/
	char/1.cc: Fix expected return type of basic_string_view::data().
	* testsuite/21_strings/basic_string_view/modifiers/remove_prefix/
	wchar_t/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/
	char/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/modifiers/remove_suffix/
	wchar_t/1.cc: Likewise.
	* testsuite/21_strings/basic_string_view/requirements/traits_neg.cc:
	New test.
	* testsuite/21_strings/basic_string_view/requirements/typedefs.cc:
	Check reference and pointer typedefs.
	* testsuite/experimental/string_view/requirements/typedefs.cc:
	Likewise.
	* testsuite/experimental/string_view/modifiers/remove_prefix/char/1.cc:
	Fix expected return type of basic_string_view::data().
	* testsuite/experimental/string_view/modifiers/remove_prefix/wchar_t/
	1.cc: Likewise.
	* testsuite/experimental/string_view/modifiers/remove_suffix/char/1.cc:
	Likewise.
	* testsuite/experimental/string_view/modifiers/remove_suffix/wchar_t/
	1.cc: Likewise.

From-SVN: r270548