[Clang] Handle structs with inner structs and no fields (#89126)

A struct that declares an inner struct, but no fields, won't have a
field count. So getting the offset of the inner struct fails. This
happens in both C and C++:

  struct foo {
    struct bar {
      int Quantizermatrix[];
    };
  };

Here 'struct foo' has no fields.

Closes: https://github.com/llvm/llvm-project/issues/88931