This adds a test for invalid characters (that would've failed before, because the printed char isn't escaped). Not sure if there's a good way to test the PrintDiagnostic code, as it appears to occur on bison parser errors, which I'm just not sure how to trigger.
Slowly bringing this into line with Bison's C++ example parser
so we can use strong semantic values for symbols rather than
leaking pointers. First step is to thread a `ParseAndLexContext`
object through the whole syntactic analysis state, like the
example has. In the example, it's called `driver`.
Co-authored-by: Geoff Romer <gromer@google.com>
Jon Meow
Dave Abrahams
