fmtsort: sort interfaces deterministically

Previously, the result of sorting a map[interface{}] containing
multiple concrete types was non-deterministic. To ensure consistent
results, sort first by type name, then by concrete value.

Fixes #30398

Change-Id: I10fd4b6a74eefbc87136853af6b2e689bc76ae9d
GitHub-Last-Rev: 1b07f0c275716e1b2834f74f9c67f897bae82882
GitHub-Pull-Request: golang/go#30406
Reviewed-on: https://go-review.googlesource.com/c/163745


Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>