Modifying inactive union member in constant expression not rejected when implicit lifetime start is disallowed

This code is the constexpr version of [Example 2 from [class.union]/5](https://eel.is/c++draft/class.union#general-example-2)

```cpp
struct X { const int a; int b; };
union Y {
X x;
int k;
constexpr Y() : x{1, 2} {}
};

constexpr int test() {
Y y;
y.k = 4; // change active member to 'k'
y.x.b = 0; // undefined behavior: y.x.b modified outside its lifetime,
// S(y.x.b) is empty because X's default constructor is deleted,
// so union member y.x's lifetime does not implicitly start
return y.x.b;
}

int main() {
constexpr int val = test(); // force constant evaluation
return val;
}
```

GCC correctly rejects this due to undefined behavior in constant evaluation, while Clang doesn't. [godbolt](https://godbolt.org/z/c47TGK6G4)

Start by compiling the reduced C++ example with Clang and GCC and compare their constant-evaluation behavior. Trace Clang's handling of the inactive union member assignment during constant evaluation; done means Clang rejects the constexpr declaration for undefined behavior, matching GCC.

Written by the indexing model from the issue text.