Merge changes I68f8d670,I122fd6bf,Iae1668a4,Ib31a0170,I56275f90, ...

* changes:
  gn2bp: add helper function to rebase relative dirs
  gn2bp: add location tag helper
  gn2bp: ensure there are not multiple occurences of arg
  gn2bp: add write_buildflag_header support to ActionSanitizer
  gn2bp: move parameter sanitization to ActionSanitizer
  gn2bp: add basic outline of ActionSanitizer with helpers