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

[via cfe-commits]

yronglin wrote:

> > Only the last commit belongs to this patch, the other commits come from dependent patches
> 
> In this case, it is better to use stacked PR.

Yeah, the dependeies PRs were merged.

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