[clang] [ObjC][Preprocessor] Handle @import directive as a pp-directive (PR #157726)

[Michael Spencer via cfe-commits]

https://github.com/Bigcheese commented:

The code changes here look fine, but it would be good to have some more test examples. Particularly around things like:
```
#define M modA
@import M;
```

and cases that shouldn't compile, like:

```
#define imp @import
imp M;
```

https://github.com/llvm/llvm-project/pull/157726