c: Add C2X BOOL_MAX and BOOL_WIDTH to limits.h

C2X adds BOOL_MAX and BOOL_WIDTH macros to <limits.h>.  As GCC only
supports values 0 and 1 for _Bool (regardless of the number of bits in
the representation, other bits are padding bits and if any of them are
nonzero, the representation is a trap representation), the values of
those macros can just be hardcoded directly in <limits.h> rather than
needing corresponding predefined macros.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/
	* glimits.h [__STDC_VERSION__ > 201710L] (BOOL_MAX, BOOL_WIDTH):
	New macros.

gcc/testsuite/
	* gcc.dg/c11-bool-limits-1.c, gcc.dg/c2x-bool-limits-1.c: New
	tests.

gcc/glimits.h

@@ -150,4 +150,12 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 # define ULLONG_WIDTH __LONG_LONG_WIDTH__
 #endif
 
+#if defined (__STDC_VERSION__) && __STDC_VERSION__ > 201710L
+/* C2X width and limit of _Bool.  */
+# undef BOOL_MAX
+# define BOOL_MAX 1
+# undef BOOL_WIDTH
+# define BOOL_WIDTH 1
+#endif
+
 #endif /* _LIMITS_H___ */

gcc/testsuite/gcc.dg/c11-bool-limits-1.c

@@ -0,0 +1,13 @@
+/* Test limits for _Bool not in <limits.h> in C11.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11" } */
+
+#include <limits.h>
+
+#ifdef BOOL_MAX
+# error "unexpected BOOL_MAX"
+#endif
+
+#ifdef BOOL_WIDTH
+# error "unexpected BOOL_WIDTH"
+#endif

gcc/testsuite/gcc.dg/c2x-bool-limits-1.c

@@ -0,0 +1,19 @@
+/* Test limits for _Bool in <limits.h> in C2x.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x" } */
+
+#include <limits.h>
+
+#ifndef BOOL_MAX
+# error "missing BOOL_MAX"
+#endif
+
+#ifndef BOOL_WIDTH
+# error "missing BOOL_WIDTH"
+#endif
+
+/* In principle _Bool can support values wider than 1 bit, stored via
+   type punning, but this is not supported by GCC.  */
+
+_Static_assert (BOOL_MAX == 1, "bad BOOL_MAX");
+_Static_assert (BOOL_WIDTH == 1, "bad BOOL_WIDTH");