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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

This change causes building libc++ in mingw mode to emit warnings.

In libc++'s own CI, it is built with `-Werror` enabled in CI, which this breaks. The upstream CI runs with a previous release of Clang, so the issue is not observed there yet, but I run a weekly test with latest Clang + libc++ tests, and that broke now: https://github.com/mstorsjo/llvm-mingw/actions/runs/35524104080/job/106113172676



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