The StackUsesFrames and FunctionCallsChangeCFA virtual functions
aren't used anywhere and aren't overridden by anything.
They were introduced when the ABISysV_ppc* code was added and weren't
used at the time. The review for the commit that added them can be
found at http://reviews.llvm.org/D5988
The commit comment notes that backtraces don't yet work:
Backtraces don't work. This is due to PowerPC ABI using a backchain pointer in memory, instead of a dedicated frame pointer register for the backchain.
So there is a possibility these were added with the intent of using
them in the future.