[ObjC][Preprocessor] Handle @import directive as a pp-directive

[yronglin]

Depends: #107168
This patch handle @import as a preprocessing directive, and since this patch, the following import directive will be ill-formed:

@import Foo\n;

Only the last commit belongs to this patch, the other commits come from dependent patches

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[yronglin]

friendly ping~

[Bigcheese]

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;

[Bigcheese]

Is this FIXME still relevant after this change?

[yronglin]

Good catch, we should remove this comments

[yronglin]

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.