Calling member function before base subobject initialization in constant evaluation not rejected

Consider this program:

```cpp
#include
struct A {
constexpr A(int x) : val(x) {}
int val;
};
struct B : A {
constexpr int f() const { return 42; }
constexpr B() : A(f()) {} // undefined behavior: calls member function f() but base A not yet initialized
};
constexpr int foo() {
constexpr B b{};
return b.val;
}
int main() {
constexpr auto x = foo();
return 0;
}
```

C++20 final draft N4861 [class.base.init]/16:
> Member functions (including virtual member functions, 11.7.2) can be called for an object under construction. Similarly, an object under construction can be the operand of the typeid operator (7.6.1.7) or of a dynamic_cast (7.6.1.6). However, if these operations are performed in a ctor-initializer (or in a function called directly or indirectly from a ctor-initializer ) before all the mem-initializer s for base classes have completed, the program has undefined behavior. [Example:
> ```cpp
> class A {
> public:
> A(int);
> };
> class B : public A {
> int j;
> public:
> int f();
> B() : A(f()), // undefined behavior: calls member function but base A not yet initialized
> j(f()) { } // well-defined: bases are all initialized
> };
> ```

So this is UB and should be rejected in constant evaluation. Neither GCC nor Clang rejects it. [godbolt](https://godbolt.org/z/Tca13GY97)

Start with the provided C++ reproducer and investigate constant-evaluation handling for constructor mem-initializers and calls made before base-class initialization. Compare current compiler behavior with the C++20 rule quoted in the issue; done means the invalid constexpr initialization is rejected with an appropriate diagnostic.

Written by the indexing model from the issue text.