Parameterise `Parser::{recover_unclosed_char,handle_missing_lit}`.

Parameterise `Parser::{recover_unclosed_char,handle_missing_lit}`.

These two methods both produce a `MetaItemLit`, and then some of the
call sites convert the `MetaItemLit` to a `token::Lit` with
`as_token_lit`.

This commit parameterises these two methods with a `mk_lit_char`
closure, which can be used to produce either `MetaItemLit` or
`token::Lit` directly as necessary.