go/types: resolve cgo base type names

go/types: resolve cgo base type names

When associating methods with their receiver base, we need to implement
the same indirection through Cgo types as is done for selector
expressions. This fixes a bug where methods declared on aliases of Cgo
types were not associated with their receiver.

While porting to types2, align the types2 testFiles helper with the
go/types implementation. In order to avoid call-site bloat, switch to an
options pattern for configuring the Config used to type-check.

Fixes golang/go#59944

Change-Id: Id14101f01c122b6c856ae5453bd00ec07e83f414
Reviewed-on: https://go-review.googlesource.com/c/go/+/493877
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>