[dev.typeparams] cmd/compile/internal/types2: add Config.AllowTypeLists to...

[dev.typeparams] cmd/compile/internal/types2: add Config.AllowTypeLists to control type list handling

Eventually the flag should not be set anymore, but we set it in the
compiler until all tests have been converted.

Also, convert some more types2 tests to use the type set notation.

Change-Id: I616599a3473451ab75d67272016b2bd3de6835af
Reviewed-on: https://go-review.googlesource.com/c/go/+/324571


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>