IRGen: Do not set dllexport on declarations.

Setting dllexport on a declaration has no effect, as we do not emit export
directives for declarations.

Part of the fix for PR32334.

Differential Revision: https://reviews.llvm.org/D31162

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@298330 91177308-0d34-0410-b5e6-96231b3b80d8