Returning from [[noreturn]] function not rejected in constant evaluation

Consider the following code:

```cpp
[[noreturn]] constexpr void f(int i) {
if (i == 0)
throw 42;
// otherwise it returns, which is UB
}

constexpr int g() {
f(1);
return 0; // unreachable
}

int main() {
static_assert(g() == 0, "This should cause a compile error due to UB");
(void)g(); // avoid warning about unused function
return 0;
}
```

From C++20 final draft N4861 [dcl.attr.noreturn]/2,
> If a function f is called where f was previously declared with the noreturn attribute and f eventually returns, the behavior is undefined.

So this code involves an undefined behavior during constant evaluation and should be rejected.

Neither GCC nor Clang rejects this. [godbolt](https://godbolt.org/z/Gjj5hv4qr)

Reproduce the issue's C++ example in Clang, focusing on the constexpr static_assert and [[noreturn]] call. Trace constant evaluation for a function that returns despite [[noreturn]], then add a regression test showing that the static_assert is rejected; done when the behavior is diagnosed consistently.

Written by the indexing model from the issue text.