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

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eaeltsin wrote:

I think the pattern is something like
```
enum { const_value = 1 };
template<size_t N> class Foo {...};
Foo<const_value + 1> foo;
```


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