Allow adding dll attributes on certain redecls with a warning if the decl hasn't been used yet (PR20746)

This shouldn't really be allowed, but it comes up in real code (see PR). As
long as the decl hasn't been used there's no technical difficulty in supporting
it, so downgrade the error to a warning.

Differential Revision: http://reviews.llvm.org/D5087

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216619 91177308-0d34-0410-b5e6-96231b3b80d8