Adds the notion of an OptionValidator.

The purpose of the OptionValidator is to determine, based on some
arbitrary set of conditions, whether or not a command option is
valid for a given debugger state.  An example of this might be
to selectively disable or enable certain command options that
don't apply to a particular platform.

This patch contains no functional change, and does not actually
make use of an OptionValidator for any purpose yet.  A follow-up
patch will begin to add the logic and users of OptionValidator.

Reviewed by: Greg Clayton, Jim Ingham

Differential Revision: http://reviews.llvm.org/D4369