Consider used attributes in hidden decls.

Without this patch we would warn and fail to output the function in the test.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193388 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index ba78928..2edebab 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -2099,6 +2099,12 @@
 /// mergeDeclAttributes - Copy attributes from the Old decl to the New one.
 void Sema::mergeDeclAttributes(NamedDecl *New, Decl *Old,
                                AvailabilityMergeKind AMK) {
+  if (UsedAttr *OldAttr = Old->getMostRecentDecl()->getAttr<UsedAttr>()) {
+    UsedAttr *NewAttr = OldAttr->clone(Context);
+    NewAttr->setInherited(true);
+    New->addAttr(NewAttr);
+  }
+
   if (!Old->hasAttrs() && !New->hasAttrs())
     return;
 
@@ -2136,6 +2142,10 @@
       }
     }
 
+    // Already handled.
+    if (isa<UsedAttr>(*i))
+      continue;
+
     if (mergeDeclAttribute(*this, New, *i, Override))
       foundAny = true;
   }