[pseudo] benchmark cleanups. NFC

- add missing benchmark for lex/preprocess steps
- name benchmarks after the function they're benchmarking, when appropriate
- remove unergonomic "run" prefixes from benchmark names
- give a useful error message if --grammar or --source are missing
- Use realistic example of how to run, run all benchmarks by default.
  (for someone who doesn't know the commands, this is the most useful action)
- Improve typos/wording in comment
- clean up unused vars
- avoid "parseable stream" name, which isn't a great name & not one I expected
  to escape from ClangPseudoMain

Differential Revision: https://reviews.llvm.org/D125312