Add support for OffsetOfExpr that contain exactly 1 component, when that component
is a field.

For example, given:

```c
struct S {
  float f;
  double d;
};
struct T {
  long l;
  int i;
  struct S s[10];
};
```

Then:
```c
offsetof(struct T, i)       // supported
offsetof(struct T, s[2].d)  // not supported currently
```