Stricter use-after-dtor detection for trivial members.

Poison trivial class members one-by-one in the reverse order of their
construction, instead of all-at-once at the very end.

For example, in the following code access to `x` from `~B` will
produce an undefined value.

struct A {
  struct B b;
  int x;
};

Reviewed By: kda

Differential Revision: https://reviews.llvm.org/D119600