[dev.typeparams] go/types: import object resolution from dev.go2go

Changes from dev.go2go:
 + Removed enableImplicitTParam
 + Fixed a bug in unpackRecv where pointer receivers were not being
   detected in the syntax. This didn't seem to actually matter, as I
   couldn't produce an incorrect test case as a result of this bug (I
   guess by the time method sets are considered, functions have already
   been type checked).
 + Updated to the new error API.
 + A line setting t.underlying to Typ[Invalid] was restored in
   Checker.validType when a cycle is detected. Though this didn't seem
   to matter, it preserves an invariant that invalid types are used to
   suppress error reporting.

Change-Id: I3b53b35368c244d67571f23d70fb991af50db540
Reviewed-on: https://go-review.googlesource.com/c/go/+/278595


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