Adds a DW_OP_call_frame_cfa handler when evaluating DWARF 3/4 expressions

in LLDB that load the canonical frame address rather than a location list.

- Handles the simple case where a CFA can be pulled from the current stack frame.
- Fixes more than one hundred failing tests with gcc 4.8!

TODO: Use UnwindPlan::GetRowForFunctionOffset if the DWARFExpression needs
to be evaluated in a context analogous to a virtual unwind (perhaps using RegisterContextLLDB).

- Also adds some comments to DWARFCallFrameInfo whenever I got confused.

llvm-svn: 187361