This change improves upon cycle detection by taking into account
cycles involving constants, variables, _and_ types. All new code
(except for the additional tests) is guarded by the useCycleMarking
(internal) flag and thus can be disabled on short notice if it
introduced new problems. (The intent is to remove this flag shortly
after 1.11 is released.)

The test suite has been extended with various additional (and mostly
esoteric) test cases which now correctly report cycles. A handful of
existing test cases now report additional errors, but those are mostly
esoteric cases. Overall, this is an improvement over the status quo.

Fixes #8699.
For #20770.

Change-Id: I6086719acd0d5200edca4a3dbe703d053496af31
Reviewed-on: https://go-review.googlesource.com/116815
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>