* add `NominalTypeDecl.isResilient`
* make the return type of `Type.getNominalFields` optional and return nil in case the nominal type is resilient.
This forces users of this API to think about what to do in case the nominal type is resilient.
Try to replace a begin_borrow with its owned operand.
This is either possible if the borrowed value (or a forwarded value if it) is copied:
```
%1 = begin_borrow %0
%2 = struct_extract %1 // a chain of forwarding instructions
%3 = copy_value %1
// ... uses of %3
end_borrow %1
```
->
```
%1 = copy_value %0
%3 = destructure_struct %0 // owned version of the forwarding instructions
// ... uses of %3
```
Or if the borrowed value is destroyed immediately after the borrow scope:
```
%1 = begin_borrow %0
%2 = struct_extract %1 // a chain of forwarding instructions
// ... uses of %2
end_borrow %1
destroy_value %1 // the only other use of %0 beside begin_borrow
```
->
```
%2 = destructure_struct %0 // owned version of the forwarding instructions
// ... uses of %2
destroy_value %2
```
