Before, we would always have a `Some` ClosureRegionRequirements if we
were inferring values for a closure. Now we only do is it has a
non-empty set of outlives requirements.

This allows us to re-use the `normalize` method on `TypeCheck`, which
is important since normalization may create fresh region
variables. This is not an ideal solution, though, since the current
representation of "liveness constraints" (a vector of (region, point)
pairs) is rather inefficient. Could do somewhat better by converting
to indices, but it'd still be less good than the older code. Unclear
how important this is.

In the future, `check_type_tests` will also potentially propagate
constriants to its caller.

The existing flags did not consider `'static` to be "free". This then
fed into what was "erasable" -- but `'static` is most certainly
erasable.

Currently, we only propagate type tests that exclude all regions from
the type.

This is needed to allow the `ClosureRegionRequirements` to capture
types that include regions.

The input/output types found in `UniversalRegions` are not normalized.
The old code used to assign them directly into the MIR, which would
lead to errors when there was a projection in a argument or return
type. This also led to some special cases in the `renumber` code.

We now renumber uniformly but then pass the input/output types into
the MIR type-checker, which equates them with the types found in MIR.
This allows us to normalize at the same time.

Turns out this works but we had no test targeting it.

The prior messages were not stable across platforms.

Only `is_local_free_region` is used.