Summary:
When there was an assignment of C struct, `x = y;`, it was translated to the statements of load and store.
```
n$0 = *y
*x = n$0
```
However, this is incorrect in Sil, because a struct is not a value that can be assigned to registers. This diff fixes the translation as assignments of each field values :
```
n$0 = *y.field1
*x.field1 = n$0
n$0 = *y.field2
*x.field2 = n$0
...
```
It copies field values of C structs on:
* assign statement
* return statement
* declarations.
It supports nested structs.
Reviewed By: jvillard
Differential Revision: D25952894
fbshipit-source-id: 355f8db9c
Sungkeun Cho