Missing diagnostic for use of non-literal type as argument to undefined constructor

When visiting a `CXXConstructExpr` during constant evaluation, we [never visit argument expressions](https://github.com/llvm/llvm-project/blob/c9c1ed7e41b245d15ee339b51696c546d5dc6d18/clang/lib/AST/ExprConstant.cpp#L11746-L11747) if the constructor itself is not suitable for constant evaluation. Consequently (when compiling with `--std=c++20` or earlier), we fail to diagnose non-literal types that appear in argument expressions for calls to such constructors ([Godbolt](https://godbolt.org/z/qeqGsreTf)).

```cpp
struct NonLiteral {
NonLiteral() {}
};

struct Swallow {
template
constexpr Swallow(T);
};

constexpr int g() { Swallow s(NonLiteral{}); return 42; }

template
constexpr Swallow::Swallow(T) {}

int main() { return g(); }
```

Start in clang/lib/AST/ExprConstant.cpp at the CXXConstructExpr handling linked in the issue, then run the supplied C++20 reproducer. Trace how argument expressions are visited when the constructor is not suitable for constant evaluation. Done means the non-literal argument is diagnosed under C++20 and earlier without regressing valid constant evaluation.

Written by the indexing model from the issue text.