DebugInfo: Simplify solution to avoid DW_AT_artificial on inlined parameters.

Originally committed in r207717, I clearly didn't look very closely at
the code to understand how existing things were working...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209680 91177308-0d34-0410-b5e6-96231b3b80d8