[Clang] Fix building with Clang < 3.9.

This is a workaround for a bug in older versions of Clang when. The
constructor that is supposed to allow for Derived to Base conversion
does not work. Remove this if we drop support for such configurations.

llvm/utils/TableGen/OptParserEmitter.cpp

@@ -110,7 +110,11 @@ class MarshallingFlagInfo final : public MarshallingKindInfo {
   static std::unique_ptr<MarshallingKindInfo> create(const Record &R) {
     std::unique_ptr<MarshallingFlagInfo> Ret(new MarshallingFlagInfo(R));
     Ret->IsPositive = R.getValueAsBit("IsPositive");
-    return Ret;
+    // FIXME: This is a workaround for a bug in older versions of libstdc++ when
+    //   compiled with Clang. The constructor that is supposed to allow for
+    //   Derived to Base conversion does not work. Remove this if we drop
+    //   support for such configurations.
+    return std::unique_ptr<MarshallingKindInfo>(Ret.release());
   }
 
 private:
@@ -204,7 +208,11 @@ struct SimpleEnumValueTable {
              "values");
     }
 
-    return Ret;
+    // FIXME: This is a workaround for a bug in older versions of libstdc++ when
+    //   compiled with Clang. The constructor that is supposed to allow for
+    //   Derived to Base conversion does not work. Remove this if we drop
+    //   support for such configurations.
+    return std::unique_ptr<MarshallingKindInfo>(Ret.release());
   }
 
 private: