[clang] [Clang](NFC) Rewrite test/SemaCXX/dllimport.cpp as done for dllexport.cpp (PR #191391)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem approved this pull request.

lgtm, this is a nice improvement. Thanks!

(I skimmed a bit, but everything I saw looked good.)

https://github.com/llvm/llvm-project/pull/191391