Don't warn on unused member functions that are extern because of a typedef.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171267 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/Sema.cpp b/lib/Sema/Sema.cpp
index 0c5e8db..a639d3b 100644
--- a/lib/Sema/Sema.cpp
+++ b/lib/Sema/Sema.cpp
@@ -363,6 +363,9 @@
       return !SemaRef->ShouldWarnIfUnusedFileScopedDecl(DeclToCheck);
   }
 
+  if (D->getLinkage() == ExternalLinkage)
+    return true;
+
   return false;
 }