[clang] [clang] Fixed inconsistent diagnostics when initializing a constexpr variable with a compound literal (PR #232825)

[Abdulmalek Almkainzi via cfe-commits]

aalmkainzi wrote:

currently in main, the second constexpr fails.

```C
const int a = 1;
constexpr int b = (int){a};

int main()
{
    constexpr int b = (int){a};
}
```

The difference stems from the special handling of file scope compound literal. their initializers are converted to constexpr

https://github.com/llvm/llvm-project/pull/232825