Reimplemented the code that backed the "settings" in lldb. There were many...

Reimplemented the code that backed the "settings" in lldb. There were many issues with the previous implementation:
- no setting auto completion
- very manual and error prone way of getting/setting variables
- tons of code duplication
- useless instance names for processes, threads

Now settings can easily be defined like option values. The new settings makes use of the "OptionValue" classes so we can re-use the option value code that we use to set settings in command options. No more instances, just "does the right thing".

llvm-svn: 162366