When importing Objective-C protocols, mark them as having external decls.
When importing Objective-C protocols, mark them as having external decls.

We already do this for Objective-C interfaces, but we never handled protocols
because the DWARF didn't represent them.  Nowadays, though, we can import them
from modules, and we have to mark them properly.

<rdar://problem/24193009>


git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@260445 91177308-0d34-0410-b5e6-96231b3b80d8

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