Update for LLVM API change to make Small(Ptr)Set::insert return pair<iterator, bool> as per the C++ standard's associative container concept.

llvm-svn: 222335
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 75ac2c34..a783183 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -143,7 +143,7 @@
 
       ~RedeclarableResult() {
         if (FirstID && Owning && isRedeclarableDeclKind(DeclKind) &&
-            Reader.PendingDeclChainsKnown.insert(FirstID))
+            Reader.PendingDeclChainsKnown.insert(FirstID).second)
           Reader.PendingDeclChains.push_back(FirstID);
       }