Because it can be used for a lifetime or a label.

…r literal.

Currently the parser will interpret any label/lifetime in certain
positions as a mistyped char literal, on the assumption that the
trailing single quote was accidentally omitted. This is reasonable for a
something like 'a (because 'a' would be valid) but not reasonable for a
something like 'abc (because 'abc' is not valid).

This commit restricts this behaviour only to labels/lifetimes that would
be valid char literals, via the new `could_be_unclosed_char_literal`
function. The commit also augments the `label-is-actually-char.rs` test
in a couple of ways:
- Adds testing of labels/lifetimes with identifiers longer than one
  char, e.g. 'abc.
- Adds a new match with simpler patterns, because the
  `recover_unclosed_char` call in `parse_pat_with_range_pat` was not
  being exercised (in this test or any other ui tests).

Fixes rust-lang#120397, an assertion failure, which was caused by this behaviour
in the parser interacting with some new stricter char literal checking
added in rust-lang#120329.