[clang] [Clang] Warn about ignored dllimport on explicit instantiations (PR #191392)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem commented:

Sorry for the delay. This looks reasonable to me.

Would you mind sending the "Rewrite test/SemaCXX/dllimport.cpp as done for dllexport.cpp" commit as a separate PR? That would reduce the size of this one and make it easier to work with.

https://github.com/llvm/llvm-project/pull/191392