The C++ committee has confirmed that passing a null pointer to the
unary basic_string_view constructor is undefined. This removes the check
from our implementation, and adds the nonnull attribute to warn when the
compiler can detect undefined input.

	* include/std/string_view (basic_string_view(const CharT*)): Remove
	check for null pointer and add nonnull attribute.
	(compare(const CharT*), compare(size_type, size_type, const CharT*))
	(find(const CharT*, size_type), rfind(const CharT*, size_type))
	(find_first_of(const CharT*, size_type))
	(find_last_of(const CharT*, size_type))
	(find_first_not_of(const CharT*, size_type))
	(find_last_not_of(const CharT*, size_type)): Add nonnull attribute.
	* testsuite/21_strings/basic_string_view/cons/char/nonnull.cc: New.
	* testsuite/21_strings/basic_string_view/operations/compare/char/
	nonnull.cc: New.
	* testsuite/21_strings/basic_string_view/operations/find/char/
	nonnull.cc: New.
	* testsuite/21_strings/basic_string_view/operations/rfind/char/
	nonnull.cc: New.

From-SVN: r261638