[clang] Fix __builtin_object_size calculation for references of unknown origin in C++23 (PR #157778)

[Akira Hatanaka via cfe-commits]

ahatanak wrote:

Shouldn't it return false even in contexts that don't require constant evaluation?

The following function (adapted from `test/AST/ByteCode/builtin-object-size-codegen.cpp`) returns 16 because `determineEndOffset` uses a conservative lower bound. gcc returns 32.

```
#include <cstdlib>

int foo() {
  struct A { char buf[16]; };
  struct B : A {};
  struct C { int i; B bs[1]; } *c = (C*)malloc(sizeof(C) + sizeof(B));

  int gi;
  gi = __builtin_object_size(&c->bs[0], 3);
  return gi;
}
```

This is exactly the case where the underlying object could become known later.

https://github.com/llvm/llvm-project/pull/157778