[clang] [clang] Check constexpr int->enum conversions consistently. (PR #143034)

[via cfe-commits]

eaeltsin wrote:

Sorry, I over-generalized :)

Here is the proper sample:
```
template< typename T, T N >
struct IC
{
  static const T next_value = static_cast<T>(N + 1);
  typedef IC< T, next_value > next;
};

enum E
{
  integral_to_integral,
  integral_to_float,
  float_to_integral,
  float_to_float
};

IC<E, float_to_float> foo;
```


https://github.com/llvm/llvm-project/pull/143034