Refactor parsing of option lists with a raw string suffix.

Summary:
A subset of the LLDB commands follows this command line interface style:
   <command name> [arguments] -- <string suffix>
The parsing code for this interface has been so far been duplicated into the different
command objects which makes it hard to maintain and reuse elsewhere.

This patches improves the situation by adding a OptionsWithRaw class that centralizes
the parsing logic and allows easier testing. The different commands now just call this class to
extract the arguments and the raw suffix from the provided user input.

Reviewers: jingham

Reviewed By: jingham

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D49106

llvm-svn: 336723