libstdc++: Disable std::string{,_view} construction from nullptr (P2166R1)

libstdc++: Disable std::string{,_view} construction from nullptr (P2166R1)

Implement this C++23 feature. Because construction from a null pointer
is undefined, we can implement it for C++11 and up, turning undefined
behaviour into a compilation error.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string(nullptr_t)): Define
	as deleted.
	(operator=(nullptr_t)): Likewise.
	* include/bits/cow_string.h (basic_string(nullptr_t)): Likewise.
	(operator=(nullptr_t)): Likewise.
	* include/std/string_view (basic_string_view(nullptr_t)):
	Likewise.
	* testsuite/21_strings/basic_string/cons/char/nullptr.cc: New test.
	* testsuite/21_strings/basic_string_view/cons/char/nonnull.cc:
	Change dg-warning to dg-error.
	* testsuite/21_strings/basic_string_view/cons/wchar_t/nonnull.cc:
	Likewise.