Remove 'XFAIL: powerpc64' from a debuginfo test

This test started XPASSing with r352467, and the change in behaviour
performed by that patch does appear to fix the cause of the original XFAIL
(missing FrameIndex DBG_VALUE), which I've replicated locally with
-mtriple=powerpc64--.

I'll write this up in PR21881 which documents the XFAIL, and seek
confirmation I haven't overlooked something here.

llvm-svn: 352471