Merging r216619:
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r216619 | hans | 2014-08-27 14:27:40 -0700 (Wed, 27 Aug 2014) | 7 lines

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
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git-svn-id: https://llvm.org/svn/llvm-project/cfe/branches/release_35@218644 91177308-0d34-0410-b5e6-96231b3b80d8