Feature request: Allow constexpr in C #1095

Currently, the allow_constexpr option is available in C++ mode but not in C mode. The constexpr specifier is also available in C since C23.¹² So I think it would be better if this option is available in C mode as well. Because this can be typed, unlike the #define directive.

#include <stdio.h>

#define FOO 10

int main(void) {
  printf("%d\n", FOO);
  return 0;
}

In C23 this code can be written as:

#include <stdio.h>

constexpr int FOO = 10;

int main(void) {
  printf("%d\n", FOO);
  return 0;
}