Both refutable and irrefutable is-pattern matches affect definite assignment:
- an irrefutable match leaves pattern variables definitely assigned
- a refutable match is an error
Fixes #25890

Also, we check to see if a constant input makes the result refutable or irrefutable:
- If a constant cannot match the pattern, we issue a warning
- If a constant always matches a constant pattern, we issue a warning
The latter two are subject to compat council approval.
Fixes #16099