Extend the unknowns-are-same-as-known-inputs type resolution heuristic.

For a very long time, one of the parser's heuristics for resolving
ambiguous operator calls has been to assume that unknown-type literals are
of the same type as the other input (if it's known).  However, this was
only used in the first step of quickly checking for an exact-types match,
and thus did not help in resolving matches that require coercion, such as
matches to polymorphic operators.  As we add more polymorphic operators,
this becomes more of a problem.  This patch adds another use of the same
heuristic as a last-ditch check before failing to resolve an ambiguous
operator or function call.  In particular this will let us define the range
inclusion operator in a less limited way (to come in a follow-on patch).