c++: P2513R4, char8_t Compatibility and Portability Fix [PR106656]

c++: P2513R4, char8_t Compatibility and Portability Fix [PR106656]

P0482R6, which added char8_t, didn't allow

  const char arr[] = u8"howdy";

because it said "Declarations of arrays of char may currently be initialized
with UTF-8 string literals. Under this proposal, such initializations would
become ill-formed."  This caused too many issues, so P2513R4 alleviates some
of those compatibility problems.  In particular, "Arrays of char or unsigned
char may now be initialized with a UTF-8 string literal."  This restriction
has been lifted for initialization only, not implicit conversions.  Also,
my reading is that 'signed char' was excluded from the allowable conversions.

This is supposed to be treated as a DR in C++20.

	PR c++/106656

gcc/c-family/ChangeLog:

	* c-cppbuiltin.cc (c_cpp_builtins): Update value of __cpp_char8_t
	for C++20.

gcc/cp/ChangeLog:

	* typeck2.cc (array_string_literal_compatible_p): Allow
	initializing arrays of char or unsigned char by a UTF-8 string literal.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/feat-cxx2b.C: Adjust.
	* g++.dg/cpp2a/feat-cxx2a.C: Likewise.
	* g++.dg/ext/char8_t-feature-test-macro-2.C: Likewise.
	* g++.dg/ext/char8_t-init-2.C: Likewise.
	* g++.dg/cpp2a/char8_t3.C: New test.
	* g++.dg/cpp2a/char8_t4.C: New test.