Add support for Bison versions up to 3.7

Add support for Bison versions up to 3.7

This requires copying the `YY_CAST` and `YY_ATTRIBUTE_UNUSED` macros
from the generated code, and extracting the `yysymbol_kind_t` enum if
it's defined.

We must also handle token type names with escaped double-quotes in
them, since it now names the `YYEOF` and `YYUNDEF` tokens `"end of
file"` and `"invalid token"` instead of `$end` and `$undefined`,
respectively.