Bounds checking for pointer dereferences and array subscripts

[secure-sw-dev-bot]

This issue was copied from https://github.com/microsoft/checkedc-clang/issues/1176

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This PR updates bounds validation to handle the following cases:

1. Validate the bounds of a pointer dereference *p or array subscript p[i].
2. Account for uses of pointer dereferences and array subscripts in observed bounds

The behavior for synthesizing member expressions whose bounds depend on a member expression that is being modified via an assignment has been extended: we now synthesize member expressions whose bounds depend on an lvalue expression that uses a member expression to update memory via an assignment. For example:

struct S {
  _Array_ptr<int> f : count(*ptr_to_len);
  _Array_ptr<int> ptr_to_len : count(10);
};

void f(struct S *s) {
  *s->ptr_to_len = 0;
}

The lvalue expression *s->ptr_to_len uses the member expression s->ptr_to_len to write to memory. We synthesize the member expression s->f whose declared bounds bounds(s->f, s->f + *s->ptr_to_len) depend on *s->ptr_to_len.

There is also a minor fix included in this PR that the pointer bounds checking depends on: in PreorderAST, the canonical form of e1[e2] is now *(e1 + e2 + 0) rather than *(e1 + e2). This enables bounds validation to treat expressions such as p[i] and *(p + i) as equivalent, since PreorderAST canonicalizes *(p + i) to *(p + i + 0).

[secure-sw-dev-bot]

Comment from @mgrang:

LGTM.