test/CodeGenCXX/dllexport.cpp: don't instantiate templates in USEMEMFUNC macro

Use the derived class in the macro instead, to make it unambigous where
the template is being instantiated.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@216142 91177308-0d34-0410-b5e6-96231b3b80d8