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

[via cfe-commits]

yronglin wrote:

> 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;
> ```

Thanks for the review, I'll add more test.

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