cmd/compile/internal/gc: support invalid types/constants in binary export data

(Corresponding x/tools/go/gcimporter change is https://go-review.googlesource.com/#/c/20827/)

Change-Id: I64e7fee2e273d387f1c51b87986294489978d250
Reviewed-on: https://go-review.googlesource.com/20828
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/bexport.go

@@ -1190,6 +1190,7 @@ const (
 	complexTag
 	stringTag
 	nilTag
+	unknownTag // not used by gc (only appears in packages with errors)
 )
 
 // Debugging support.
@@ -1218,6 +1219,8 @@ var tagString = [...]string{
 	-fractionTag: "fraction",
 	-complexTag:  "complex",
 	-stringTag:   "string",
+	-nilTag:      "nil",
+	-unknownTag:  "unknown",
 }
 
 // untype returns the "pseudo" untyped type for a Ctype (import/export use only).
@@ -1289,6 +1292,9 @@ func predeclared() []*Type {
 			// package unsafe
 			Types[TUNSAFEPTR],
 
+			// invalid type (package contains errors)
+			Types[Txxx],
+
 			// any type, for builtin export data
 			Types[TANY],
 		}

src/cmd/compile/internal/gc/bimport.go

@@ -509,6 +509,9 @@ func (p *importer) value(typ *Type) (x Val) {
 	case stringTag:
 		x.U = p.string()
 
+	case unknownTag:
+		Fatalf("importer: unknown constant (importing package with errors)")
+
 	case nilTag:
 		x.U = new(NilVal)
 

src/go/internal/gcimporter/bimport.go

@@ -681,7 +681,10 @@ var predeclared = []types.Type{
 	// package unsafe
 	types.Typ[types.UnsafePointer],
 
-	// any type, for builtin export data
+	// 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],
 }