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

Removing the `isComplete` check altogether leads to cycles in,
for instatnce, F-bound type parameters:

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

But, I believe that we can (and must) eagerly initialize the type
parameter symbols if we are typechecking a pattern.

While this appeared to regress in 2.11.x, but the problem was in fact
dormant and was merely uncovered in the fix for SI-7756, 3df1d77fc.