In several cases this avoids the need to clone the underlying pattern, and then
print the clone later.

This hides the fact that we print `WitnessPat` by converting it to `thir::Pat`
and then printing that.

This gives a clearer view of the (diagnostic) code that expects to be able to
print THIR patterns, and makes it possible to experiment with requiring some
kind of context (for ID lookup) when printing patterns.