libcpp: Use [[likely]] conditionally

Let's hide [[likely]] behind a macro, to suppress warnings if the
compiler doesn't support it.

Co-authored-by: Jonathan Wakely <jwakely@redhat.com>

	PR preprocessor/103355

libcpp/ChangeLog:

	* lex.c: Use ATTR_LIKELY instead of [[likely]].
	* system.h (ATTR_LIKELY): Define.

libcpp/lex.c

@@ -1286,7 +1286,7 @@ namespace bidi {
       case kind::RTL:
 	/* These aren't popped by a PDF/PDI.  */
 	break;
-      [[likely]] case kind::NONE:
+      ATTR_LIKELY case kind::NONE:
 	break;
       default:
 	abort ();

libcpp/system.h

@@ -422,6 +422,16 @@ extern void fancy_abort (const char *, int, const char *) ATTRIBUTE_NORETURN;
 #define gcc_checking_assert(EXPR) ((void)(0 && (EXPR)))
 #endif
 
+#ifdef __has_cpp_attribute
+# if __has_cpp_attribute(likely)
+#  define ATTR_LIKELY [[likely]]
+# elif __has_cpp_attribute(__likely__)
+#  define ATTR_LIKELY [[__likely__]]
+# else
+#  define ATTR_LIKELY
+# endif
+#endif
+
 /* Poison identifiers we do not want to use.  */
 #if (GCC_VERSION >= 3000)
 #undef calloc