Decoupled Options from CommandInterpreter.

Options used to store a reference to the CommandInterpreter instance
in the base Options class.  This made it impossible to parse options
independent of a CommandInterpreter.

This change removes the reference from the base class.  Instead, it
modifies the options-parsing-related methods to take an
ExecutionContext pointer, which the options may inspect if they need
to do so.

Closes https://reviews.llvm.org/D23416
Reviewers: clayborg, jingham

llvm-svn: 278440