go/loader: Initialize (types/Info).Instances field

Initialize the Instances field of PackageInfo.Info during importing.

Needed for go/ssa and similar users.

Updates golang/go#48525

Change-Id: Ibacf925e677ec6e90068b90a4f381d96c22338cc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/385774
Run-TryBot: Tim King <taking@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>

go/loader/loader.go

@@ -23,6 +23,7 @@ import (
 
 	"golang.org/x/tools/go/ast/astutil"
 	"golang.org/x/tools/go/internal/cgo"
+	"golang.org/x/tools/internal/typeparams"
 )
 
 var ignoreVendor build.ImportMode
@@ -1053,6 +1054,7 @@ func (imp *importer) newPackageInfo(path, dir string) *PackageInfo {
 		errorFunc: imp.conf.TypeChecker.Error,
 		dir:       dir,
 	}
+	typeparams.InitInstanceInfo(&info.Info)
 
 	// Copy the types.Config so we can vary it across PackageInfos.
 	tc := imp.conf.TypeChecker