Simplify FuncUnwinders::GetEHFrameAugmentedUnwindPlan

Summary:
GetEHFrameAugmentedUnwindPlan duplicated the work of GetEHFrameUnwindPlan in getting the original
plan from DWARF CFI. This changes the function to call GetEHFrameUnwindPlan instead of doing all
the work itself. A copy constructor is added to UnwindPlan to enable plan copying.

Test Plan: No regressions on linux test suite.

Reviewers: jasonmolenda, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9369

llvm-svn: 236607