go/importer: match predeclared type list with gc's list in binary exporter

I think we had this code before but it may have gone lost somehow.

Change-Id: Ifde490e686de0d2bfe907cbe19c9197f24f5fa8e
Reviewed-on: https://go-review.googlesource.com/21537Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/go/internal/gcimporter/bimport.go

@@ -723,7 +723,11 @@ var predeclared = []types.Type{
 	// invalid type
 	types.Typ[types.Invalid], // only appears in packages with errors
 
-	// TODO(mdempsky): Provide an actual Type value to represent "any"?
-	// (Why exactly does gc emit the "any" type?)
-	types.Typ[types.Invalid],
+	// used internally by gc; never used by this package or in .a files
+	anyType{},
 }
+
+type anyType struct{}
+
+func (t anyType) Underlying() types.Type { return t }
+func (t anyType) String() string         { return "any" }