[Bug c/115768] New: [C23] constexpr array of string literals not optimized

[biggs at biggs dot xyz via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115768

            Bug ID: 115768
           Summary: [C23] constexpr array of string literals not optimized
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: biggs at biggs dot xyz
  Target Milestone: ---

Consider:

https://godbolt.org/z/c9fME1qdh
```c
#include <stdio.h>

enum ColorEnum {
    CE_RED,
    CE_GREEN,
    CE_BLUE,
};

static constexpr char names[][6] = {
    [CE_RED] = "RED",
    [CE_GREEN] = "GREEN",
    [CE_BLUE] = "BLUE",
};

typedef struct Color Color;

struct Color {
    enum ColorEnum value;
};

[[maybe_unused]] static constexpr Color RED = { CE_RED };
[[maybe_unused]] static constexpr Color GREEN = { CE_GREEN };
[[maybe_unused]] static constexpr Color BLUE = { CE_BLUE };

int main() {
    puts(names[RED.value]);
}
```

vs.

https://godbolt.org/z/MTWWzzh17
```c
#include <stdio.h>

enum ColorEnum {
    CE_RED,
    CE_GREEN,
    CE_BLUE,
    CE_SIZE
};

static char const*const names[CE_SIZE] = {
    [CE_RED] = "RED",
    [CE_GREEN] = "GREEN",
    [CE_BLUE] = "BLUE",
};

typedef struct Color Color;

struct Color {
    enum ColorEnum value;
};

[[maybe_unused]] static constexpr Color RED = { CE_RED };
[[maybe_unused]] static constexpr Color GREEN = { CE_GREEN };
[[maybe_unused]] static constexpr Color BLUE = { CE_BLUE };

int main() {
    puts(names[RED.value]);
}
```

The unused constexpr names aren't optimized out but the const*const ones are.