UnwindPlan::Row refactor -- add support for CFA set by a DWARF expression

Summary:
This change refactors UnwindPlan::Row to be able to store the fact that the CFA is value is set
by evaluating a dwarf expression (DW_CFA_def_cfa_expression). This is achieved by creating a new
class CFAValue and moving all CFA setting/getting code there. Note that code using the new
CFAValue::isDWARFExpression is not yet present and will be added in a follow-up patch. Therefore,
this patch should not change the functionality in any way.

Test Plan: Ran tests on Mac and Linux. No regressions detected.

Reviewers: jasonmolenda, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D7755

llvm-svn: 230210