Rename unreachable_lines to skipped_lines (#15483)
In #15164 we've made it so that `# type: ignore`s in lines skipped in
semantic analysis would not be flagged as unused.
In #15164 they were called "unreachable" lines but unreachability during
type-checking can result in the "Statement is unreachable" error, while
these statements are pruned before type-checking, so for clarity we'll
use the term "skipped".
