go/types, types2: more readable inference trace

go/types, types2: more readable inference trace

Print the unification mode in human-readable form.
Use a tab and // instead of ()'s to show unification mode
and whether operands where swapped.

These changes only affect inference trace output, which is
disabled by default. For easier debugging.

For #60933.

Change-Id: I95299c6e09b90670fc45addc4f9196b6cdd3b59f
Reviewed-on: https://go-review.googlesource.com/c/go/+/505395
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>