DwarfDebug: Don't set frame index locations on abstract variables.

Abstract variables should never have/use locations. In this case the
data wasn't used, so no functional change intended here, just
simplification.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208820 91177308-0d34-0410-b5e6-96231b3b80d8