[SE-0408] Add `where` clause support

[simanerush]

Description

func test<each T: Collection>(ts: repeat each T) {
  for t in repeat each ts where !t.isEmpty {
    // code
  }
}

The above code should compile and t should be a non-empty collection.

Additional information

The problem is that the where clause is type-checked together with the pattern. We can only get the opened element environment once that's done. Thus, we cannot guarantee that we have it available when the where clause is type-checked.