SI-8023 Fix symbol-completion-order bug of type var patterns

Removing the `isComplete` check altogether leads to cycles in
simple cases like:

  trait LSO[+A, +Repr <: LSO[A, Repr]] // error: illegal cyclic reference involving type Repr

But, I believe that we can eagerly initialize the type parameter
symbols if we are typechecking a pattern, as in the reported bug.