narrowing conversion in list initialization is not flagged when using conversion function to convert to reference

```cpp
#include
#include

struct Big
{
    operator uint64_t()
    {
        return a;
    }
    uint64_t a;
};

struct Small
{
    uint32_t a;
    using param_t = const uint32_t&; // compiles
    // using param_t = const uint32_t; but if its non ref doesnt compile
    explicit Small(param_t a) : a(a) {};
};

void foo()
{
    int a;
    std::cin >> a;
    Big big(a);
    Small small{big};
    // Small small2{a}; does not compile even with uint32_t&
}
```

clang does not issue a diagnostic on narrowing conversion when during a conversion sequence with a conversion function the parameter is reference type
in the code above, if the ref qualifier is removed then narrowing conversion is flagged

gcc on the other hand issues a diagnostic (warning) in all cases

https://godbolt.org/z/b79T5jo8K

i think this is the relevant quote from the standard https://eel.is/c++draft/dcl.init.list#3.7
> Otherwise, if T is a class type, constructors are considered ... If a narrowing conversion (see below) is required to convert any of the arguments, the program is ill-formed

Start with the reduced C++ reproducer in the issue and compare Clang's behavior with the cited [dcl.init.list]#3.7 wording and GCC. Trace list-initialization overload and conversion handling when a conversion function feeds a reference parameter, then add a regression test covering both parameter forms. Done means Clang diagnoses the narrowing conversion consistently.

Written by the indexing model from the issue text.