[llvm-locstats][NFC] Support OOP concept

Making these changes, the code becomes more robust and easier for
adding the new features.

  -Introduce the LocationStats class representing the statistics
  -Add the pretty_print() method in the LocationStats class
  -Add additional '-' for the program options
  -Add the verify_program_inputs() function
  -Add the parse_locstats() function
  -Rename 'results' => 'opts'
  -Add more comments

Differential Revision: https://reviews.llvm.org/D71868