[CIR][CodeGen] Add support for non-convertible function prototypes in virtual table lowering

Minimized test case:
```c++
#include <sstream>
class A {
  std::ostream a;
  A();
};
class B : public std::basic_stringbuf<char> {};
A::A() : a(new B) {}
```

Failed assertion:
https://github.com/llvm/clangir/blob/aeac352c9de907fcbb7adde22bbe4a7cfa3105be/clang/lib/CIR/CodeGen/CIRGenCall.cpp#L121

Flags: `-fclangir -O2 `