[BoundsSafety] Add `-fexperimental-bounds-safety` CC1 and language option and use it to tweak `counted_by`'s semantics (#92623)

This adds the `-fexperimental-bounds-safety` cc1 and corresponding
language option. This language option enables "-fbounds-safety" which is
a bounds-safety extension for C that is being incrementally upstreamed.

This cc1 flag is not exposed as a driver flag yet because most of the
implementation isn't upstream yet.

The language option is used to make a small semantic change to how the
`counted_by` attribute is treated. Without
`-fexperimental-bounds-safety` the attribute is allowed (but emits a
warning) on a flexible array member where the element type is a struct
with a flexible array member. With the flag this situation is an error.

E.g.

```
struct has_unannotated_FAM {
  int count;
  char buffer[];
};

struct buffer_of_structs_with_unnannotated_FAM {
  int count;
  // Forbidden with `-fexperimental-bounds-safety`
  struct has_unannotated_FAM Arr[] __counted_by(count);
};
```

rdar://125400392

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