[Dexter] Add a simple logging class to Dexter

Adds a basic logging class to Dexter that uses the existing PrettyOutput
class for printing and supports 3 levels of verbosity (note, warning,
error). Intended to consolidate the logging logic for Dexter into one
place, removing the need for conditional log statements and making it
easier for us later if we wish to use a more complete logging class.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D144983