cmd/compile/internal/importer: enable aliases

Flips the pkgReader.enableAlias flag to true when reading unified IR.
This was disabled while resolving #66873. This resolves the TODO to
flip it back to true.

Updates #66873
Updates #68778

Change-Id: Ifd52b0f9510d6bcf151de1c9a18d71ab548c14e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/604099
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

Author: timothy-king

src/cmd/compile/internal/importer/ureader.go

@@ -28,11 +28,9 @@ func ReadPackage(ctxt *types2.Context, imports map[string]*types2.Package, input
 	pr := pkgReader{
 		PkgDecoder: input,
 
-		ctxt:    ctxt,
-		imports: imports,
-		// Currently, the compiler panics when using Alias types.
-		// TODO(gri) set to true once this is fixed (issue #66873)
-		enableAlias: false,
+		ctxt:        ctxt,
+		imports:     imports,
+		enableAlias: true,
 
 		posBases: make([]*syntax.PosBase, input.NumElems(pkgbits.RelocPosBase)),
 		pkgs:     make([]*types2.Package, input.NumElems(pkgbits.RelocPkg)),