Add support for checking `library_names` in the generator auditor

[JamesWrigley]

This fixes spurious warnings when library_names is set instead of library_name.

Fixes #476.

[Gnimuc]

library_name is the fallback libname for those headers that are not captured by library_names.

test case:

# library_name = ">/+-;'[]{}()!@#$%^&*~`"
library_names = {}

[JamesWrigley]

Alrighty, I did a second attempt in 891aff4. Now the auditor will check both library_name and the lib names in library_names for validity, and it'll only log a warning if library_name is not set and library_names doesn't contain entries for all the headers. I had to modify the Audit pass and TranslationUnit a bit to make that work since we don't store the passed headers in a Context (and I figured it made sense to store the file path with its TranslationUnit).

[ViralBShah]

Thanks folks!