C: Implement C2X N2653 char8_t and UTF-8 string literal changes

This patch implements the core language and compiler dependent library
changes adopted for C2X via WG14 N2653.  The changes include:
- Change of type for UTF-8 string literals from array of const char to
  array of const char8_t (unsigned char).
- A new atomic_char8_t typedef.
- A new ATOMIC_CHAR8_T_LOCK_FREE macro defined in terms of the existing
  __GCC_ATOMIC_CHAR8_T_LOCK_FREE predefined macro.

gcc/ChangeLog:

	* ginclude/stdatomic.h (atomic_char8_t,
	ATOMIC_CHAR8_T_LOCK_FREE): New typedef and macro.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_string_literal): Use char8_t as the type
	of CPP_UTF8STRING when char8_t support is enabled.
	* c-typeck.cc (digest_init): Allow initialization of an array
	of character type by a string literal with type array of
	char8_t.

gcc/c-family/ChangeLog:

	* c-lex.cc (lex_string, lex_charconst): Use char8_t as the type
	of CPP_UTF8CHAR and CPP_UTF8STRING when char8_t support is
	enabled.
	* c-opts.cc (c_common_post_options): Set flag_char8_t if
	targeting C2x.

gcc/testsuite/ChangeLog:
	* gcc.dg/atomic/c2x-stdatomic-lockfree-char8_t.c: New test.
	* gcc.dg/atomic/gnu2x-stdatomic-lockfree-char8_t.c: New test.
	* gcc.dg/c11-utf8str-type.c: New test.
	* gcc.dg/c17-utf8str-type.c: New test.
	* gcc.dg/c2x-utf8str-type.c: New test.
	* gcc.dg/c2x-utf8str.c: New test.
	* gcc.dg/gnu2x-utf8str-type.c: New test.
	* gcc.dg/gnu2x-utf8str.c: New test.

1 files changed, 2 insertions, 2 deletions

diff --git a/gcc/c-family/c-opts.cc b/gcc/c-family/c-opts.cc
index 4e14636..1cf119a 100644
--- a/gcc/c-family/c-opts.cc
+++ b/gcc/c-family/c-opts.cc
@@ -1059,9 +1059,9 @@ c_common_post_options (const char **pfilename)
   if (flag_sized_deallocation == -1)
     flag_sized_deallocation = (cxx_dialect >= cxx14);
 
-  /* char8_t support is new in C++20.  */
+  /* char8_t support is implicitly enabled in C++20 and C2X.  */
   if (flag_char8_t == -1)
-    flag_char8_t = (cxx_dialect >= cxx20);
+    flag_char8_t = (cxx_dialect >= cxx20) || flag_isoc2x;
 
   if (flag_extern_tls_init)
     {