Clang rejects implicit conversion from an integer lvalue reference to another integer rvalue reference via a conversion operator

The following program is rejected by clang but accepted by both gcc and msvc. [Demo](https://godbolt.org/z/1ofEdhKzW)
```
#include

struct S
{
operator long &() const
{
exit(0);
}
};

int main() { int &&a((S())); }
```
Clang says:
```
:9:20: error: no viable conversion from 'S' to 'int'
9 | int main() { int &&a((S())); }
| ^ ~~~~~
:4:2: note: candidate function
4 | operator long &() const {
|
```

Start with the reduced C++ reproducer in the issue and compare Clang's diagnostic with the accepted behavior shown for GCC and MSVC in the linked Compiler Explorer demo. Trace the compiler's implicit-conversion handling for the conversion operator, then verify that the reproducer is accepted without the reported error.

Written by the indexing model from the issue text.