Test restrictions on taking addresses of members and variables.

[dtarditi]

Checked C restricts taking the addresses of:

1. Members with member bounds declarations.
2. Members used in member bounds declarations.
3. Variables with bounds declarations.
4. Variables/variable members used in bounds declarations.

This add tests of restrictions 1-3, as part of implementing checkedc/checkedc-clang#213 and checkedc/checkedc-clang#212:

• Taking the address of non-array members with or used in bounds declarations is now an error.
• Taking the address of non-array members with or used in bounds-safe interfaces is allowed in unchecked scopes. It is an error in checked scopes.
• Taking the address of non-array variables with bounds declaration is now an error.

It is OK to take the address of an array variable or member because you can't use the resulting pointer to modify the pointer that the array converts to.

The trickier cases to test involve nested members. Given

struct NestedLen {
   int len; 
};

struct S {
   struct NestedLen n;
   _Array_ptr<int> p : count(n.len);
}

we don't allow the addresses of n or n.len to be taken. However, if NestedLen is not embedded in S, we allow the address of a struct of type NestedLen to be taken.

The matching compiler change is at checkedc/checkedc-clang#490.

[awruef]

Looks good.