Fix __builtin_assume_aligned incorrect type descriptor and C++ object polymorphic address

Fix __builtin_assume_aligned incorrect type descriptor

example from @rsmith

struct A { int n; };
struct B { int n; };
struct C : A, B {};

void *f(C *c) {
  // Incorrectly returns `c` rather than the address of the B base class.
  return __builtin_assume_aligned((B*)c, 8);
}

Differential Revision: https://reviews.llvm.org/D133583