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'.
