SI-1503 don't assume unsound type for ident/literal patterns

The fix only kicks in under -Xfuture. We also warn under -Xlint.

What type should a variable bound to the value matched by a pattern have?
To avoid CCEs, it should be a type that's implied by the matching
semantics of the pattern.

Usually, the type implied by a pattern matching a certain value
is the pattern's type, because pattern matching implies instance-of checks.

However, Stable Identifier and Literal patterns are matched using `==`,
which does not imply a type for the binder that binds the matched value.

The change in type checking due to this fix is that programs that used to crash with a CCE
(because we blindly cast to the type of the pattern, which a `==` check does not imply)
now get a weaker type instead (and no cast). They may still type check, or they may not.

To compensate for this fix, change `case x@Foo => x` to `case x: Foo.type => x`,
if it's important that `x` have type `Foo.type`.

See also:
- SI-4577: matching of singleton type patterns uses `eq`,
  not `==` (so that the types are not a lie).
- SI-5024: patmat strips unused bindings, but affects semantics