[clang] [clang] reject initializing constexpr objects with compound literals that contain non-null pointers (PR #230477)

[Abdulmalek Almkainzi via cfe-commits]

aalmkainzi wrote:

The problem is that there is a difference between file scope compound literal and `constexpr`.
```C
const int a = 1;
constexpr int i = (int){a}; // ok
constexpr int j = a; // error
```
I'm looking into it a bit more. Maybe this PR is not going to be needed if I manage to fix the root issue.

https://github.com/llvm/llvm-project/pull/230477