Postfix: move more code out of the PDB plugin

Summary:
Previously we moved the code which parses a single expression out of the PDB
plugin, because that was useful for DWARF expressions in breakpad. However, FPO
programs are used in breakpad files too (when unwinding on windows), so this
completes the job, and moves the rest of the FPO parser too.

Reviewers: amccarth, aleksandr.urakov

Subscribers: aprantl, markmentovai, rnk, lldb-commits

Differential Revision: https://reviews.llvm.org/D66634

llvm-svn: 369894