Change level checking and instantiation for invariantly occurring TypeVars

Also instantiate them to their lower bounds, instead of updating the level.
It's a trial balloon.

Author: odersky

compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala

@@ -499,7 +499,7 @@ trait ConstraintHandling {
             val inst = tp.instanceOpt
             if inst.exists then apply(need, inst)
             else if tp.nestingLevel > maxLevel then
-              if variance > 0 then nestedVarsLo += tp
+              if variance >= 0 then nestedVarsLo += tp
               else if variance < 0 then nestedVarsHi += tp
               else tp.nestingLevel = maxLevel
                 // For invariant type variables, we use a different strategy.