[clang] [Clang] Warn if both of `dllexport`/`dllimport` and `exclude_from_explicit_instantiation` are specified (PR #183515)

[Tomohiro Kashiwada via cfe-commits]

kikairoya wrote:

Thanks!
Would you mind pushing the merge button for me?

https://github.com/llvm/llvm-project/pull/183515