Handle misplaced _Checked modifier for array member or variable.

[dtarditi]

It's possible to place a _Checked modifier on a declarator and have it be ignored:

// Spot check bounds for a flexible array member.
struct S5 {
  int len;
  int _Checked arr[] : count(len);
};

A programmer likely meant:

// Spot check bounds for a flexible array member.
struct S5 {
  int len;
  int arr _Checked[] : count(len);
};

We should only allow _Checked on function declarators or check that the modified type is indeed a valid checked type (for example a checked array).

[dtarditi]

This is fixed by PR #567.