[ASTImporter] Class with repeated friends are rejected when re-imported into initial empty Context

```c
struct foo { friend class X; friend class X; }; // FromContext
```
only one friend is imported, similar to the following:
```c
struct foo { friend class X; }; // ToContext
```
but when imported again, `struct foo` in FromContext is reported as
not equivalent to `struct foo` in ToContext, thus rejected.