Members are left unintialized in very large class

Came across this issue in generated code I work with. When there is a very, very large number of member variables, newer clangs seem to not initialize all members in the class. Built with `-std=c++23 -Wall -Werror`.

Clang 17, 19 init all members. Clang >=20 seem to have this issue.

E.g.

https://godbolt.org/z/s9YGK5rTr

```cpp
#include

struct W {
unsigned m;
};

#define M1(x) W x{1};
#define M4(p) M1(p##0) M1(p##1) M1(p##2) M1(p##3)
#define M16(p) M4(p##0) M4(p##1) M4(p##2) M4(p##3)
#define M64(p) M16(p##0) M16(p##1) M16(p##2) M16(p##3)
#define M256(p) M64(p##0) M64(p##1) M64(p##2) M64(p##3)
#define M1K(p) M256(p##0) M256(p##1) M256(p##2) M256(p##3)
#define M4K(p) M1K(p##0) M1K(p##1) M1K(p##2) M1K(p##3)
#define M16K(p) M4K(p##0) M4K(p##1) M4K(p##2) M4K(p##3)
#define M64K(p) M16K(p##0) M16K(p##1) M16K(p##2) M16K(p##3)

class Big {
public:
M64K(a)
unsigned value;

explicit Big(unsigned initialValue);
};

Big::Big(unsigned initialValue) : value{initialValue} {}

int main()
{
Big big{0x1234};
std::printf("previous member = %#x\n", big.a33333333.m); // All these members should be 1, right?
std::printf("value = %#x\n", big.value); // and value should be 0x1234?

return big.value == 0x1234 ? 0 : 1;
}
```

Start by compiling the provided Godbolt reproducer with -std=c++23 -Wall -Werror across Clang 17, 19, and 20 or newer. Compare whether the large class members remain initialized and whether value remains 0x1234; the issue is done when the newer Clang behavior matches the expected output without regressions.

Written by the indexing model from the issue text.