[dev.typeparams] go/types: import changes to types.Info from dev.go2go

Import changes related to tracking type inferences and sanitizing
types.Info from the dev.go2go branch. Notably, the following were all
intentionally omitted from this import:
 + types.Error.Full is not imported, due to it being a public API that
   requires some further thought.
 + The Config.AcceptMethodTypeParams, InferFromConstraints, and Trace
   flag are not imported. The expectation is that we will not accept
   method type parameters for now, will always infer from constraints,
   and will continue to use the trace constant to guard tracing.
 + Some trace annotations are not imported to from the checking pass. We
   can add them back later, but for now they seemed verbose.
 + Checker.useBrackets is removed. This is no longer configurable.

Change-Id: I7f6315d66b200c92ffd1e55c9fd425a5d99149ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/278312


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