RegisterContextLLDB: Push CFA value on DWARF stack when evaluating register expressions

Summary:
This behavior is specified in the Section 6.4.2.3 (Register Rule
instructions) of the DWARF4 spec. We were not doing that, which meant
that any register rule which was relying on the cfa value being there
was not evaluated correctly (it was aborted due to "out of bounds"
access).

I'm not sure how come this wasn't noticed before, but I guess this has
something to do with the fact that dwarf unwind expressions are not used
very often, and when they are, the situation is so complicated that the
CFA is of no use. I noticed this when I started emitting dwarf
expressions for the unwind information present in breakpad symbol files.

Reviewers: jasonmolenda, clayborg

Subscribers: aprantl, lldb-commits

Differential Revision: https://reviews.llvm.org/D61018

llvm-svn: 360158