Added a "--debug" option to the "expression" command.

Cleaned up ClangUserExpression::Evaluate() to have only one variant that takes a "const EvaluateExpressionOptions& options" instead of taking many arguments.

The "--debug" option is designed to allow you to debug your expression by stopping at the first instruction (it enables --ignore-breakpoints=true and --unwind-on-error=false) and allowing you to step through your JIT code. It needs to be more integrated with the thread plan, so I am checking this in so Jim Ingham can make it happen.