Add an alternative CFA type.

Summary:
PowerPC handles the stack chain with the current stack pointer being a pointer
to the backchain (CFA).  LLDB currently has no way of handling this, so this
adds a "CFA is dereferenced from a register" type.

Discussed with Jason Molenda, who also provided the initial patch for this.

Reviewers: jasonmolenda

Reviewed By: jasonmolenda

Subscribers: emaste, lldb-commits

Differential Revision: http://reviews.llvm.org/D6182

llvm-svn: 221788