Character Literals (#1934) (#1964)

Put character literals in single quotes, like `'a'`. Character literals work
like numeric literals:

-   Every different literal value has its own type.
-   The bit width is determined by the type of the variable the literal is
    assigned to, not the literal itself. Follows the plan from #1934.

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: josh11b <josh11b@users.noreply.github.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>