Remove warnings from -Wchar-subscripts for known positive constants (#69061)

Fixes #18763

Remove warnings when using a signed char as an array bound if the char is a known positive constant.

This goes one step farther than gcc does.

For example given the following code
```c++
char upper[300];

int main() {
  upper['a'] = 'A';
  char b = 'a';
  upper[b] = 'A';
  const char c = 'a';
  upper[c] = 'A';
  constexpr char d = 'a';
  upper[d] = 'A';
  char e = -1;
  upper[e] = 'A';
  const char f = -1;
  upper[f] = 'A';
  constexpr char g = -1;
  upper[g] = 'A';
  return 1;
}
```

clang currently gives warnings for all cases, while gcc gives warnings
for all cases except for 'a' (https://godbolt.org/z/5ahjETTv3)

With the change there is no longer any warning for 'a', 'c', or 'd'.

0 files changed, 0 insertions, 0 deletions