Revert "[clang] Don't inherit dllimport/dllexport to...

Revert "[clang] Don't inherit dllimport/dllexport to exclude_from_explicit_instantiation members during explicit instantiation (#65961)"

This uncovered a problem with virtual methods and
exclude_from_explicit_instantiation, see
https://github.com/llvm/llvm-project/issues/66909

Reverting until that's fixed.

> This is a continuation of https://reviews.llvm.org/D155713
>
> Fixes https://github.com/llvm/llvm-project/issues/40363

This reverts commit 84216d17.