Bytecode interpreter: Crash with invalid base casts

[tbaederr]

See:

struct C{
  int a;
};
struct S {
  constexpr S() {
    ((C*)this)->a = 12;
  }
};
constexpr S s;

We don't diagnose the invalid cast from S* to C* and instead crash in Pointer::isField() which is called via GetPtrBasePop().

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Timm Baeder (tbaederr)

See: ```c++ struct C{ int a; }; struct S { constexpr S() { ((C*)this)->a = 12; } }; constexpr S s; ``` We don't diagnose the invalid cast from `S*` to `C*` and instead crash in `Pointer::isField()` which is called via `GetPtrBasePop()`.