[clang] [Sema] Don't drop weak_import from a declaration that follows a declaration directly contained in a linkage-specification (PR #85886)

[John McCall via cfe-commits]

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

LGTM.  The (existing) diagnostic wording does seem a little awkward, so if you want to put effort into cleaning that up, it'd be welcome, but I won't hold up this fix.

https://github.com/llvm/llvm-project/pull/85886