SI-8237 Avoid cyclic constraints when inferring hk type args

An `AppliedTypeVars` spawned from `HKTypeVar#applyArgs`
(necessarily) shares the same `TypeConstraints`.

But, we can have multiple ATVs based on a single HKTV floating
around during inference, and they can appear on both sides
of type relations. An example of this is traced out in
the enclosed test.

This commit avoids registering upper/lower bound constraints
when this is detected.

In the enclosed test, we end up with an empty set of constraints
for `?E`, which results in inference of Nothing, which is what
we expect.

I have also improved the printing of ATVs (to include the args)
and sharpened the log message when `solve` leaves type variables
instantiated to `NoType`, rather than some other type that doesn't
conform to the bounds. Both of these changes helped me to get
to the bottom of this ticket. The improved `ATV#toString` shows
up in some updated checkfiles.

The reported test has quite a checkered history:

  - in 2.10.0 it worked, but more by good luck than good planning
  - after the fix for SI-7226 / 221f52757aa6, it started crashing
  - from 3bd897ba0054f (a merge from 2.10.x just before 2.11.0-M1)
    we started getting a type inference failure, rather than a crash.
    "no type parameters for method exists [...] because cyclic
    instantiation".
  - It still crashes in `isGround` in 2.10.3.

1 files changed, 1 insertions, 1 deletions

diff --git a/test/files/neg/names-defaults-neg.check b/test/files/neg/names-defaults-neg.check
index 880ddc4327..20ddd55f1f 100644
--- a/test/files/neg/names-defaults-neg.check
+++ b/test/files/neg/names-defaults-neg.check
@@ -88,7 +88,7 @@ names-defaults-neg.scala:76: error: no type parameters for method test4: (x: T[T
  --- because ---
 argument expression's type is not compatible with formal parameter type;
  found   : List[Int]
- required: ?T
+ required: ?T[?T[List[?T[X forSome { type X }]]]]
 Error occurred in an application involving default arguments.
   test4()
   ^