godoc: use "IsPredeclared" of go/doc

In the go/doc, the same map as "predeclared" is defined.
Instead of the "predeclared" map use the "IsPredeclared" function
which is an accessor for the go/doc map.

Deleted unnecessary "predeclared".

Fixes golang/go#20357

Change-Id: I4ea360efddd28a9a1236d5adfdafb1a0970a4215
Reviewed-on: https://go-review.googlesource.com/58890
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

godoc/linkify.go

@@ -13,6 +13,7 @@ package godoc
 import (
 	"fmt"
 	"go/ast"
+	"go/doc"
 	"go/token"
 	"io"
 	"strconv"
@@ -182,7 +183,7 @@ func linksFor(node ast.Node) (links []link) {
 				links = append(links, l)
 			} else {
 				l := link{name: n.Name}
-				if n.Obj == nil && predeclared[n.Name] {
+				if n.Obj == nil && doc.IsPredeclared(n.Name) {
 					l.path = builtinPkgPath
 				}
 				links = append(links, l)
@@ -192,49 +193,3 @@ func linksFor(node ast.Node) (links []link) {
 	})
 	return
 }
-
-// The predeclared map represents the set of all predeclared identifiers.
-// TODO(gri) This information is also encoded in similar maps in go/doc,
-//           but not exported. Consider exporting an accessor and using
-//           it instead.
-var predeclared = map[string]bool{
-	"bool":       true,
-	"byte":       true,
-	"complex64":  true,
-	"complex128": true,
-	"error":      true,
-	"float32":    true,
-	"float64":    true,
-	"int":        true,
-	"int8":       true,
-	"int16":      true,
-	"int32":      true,
-	"int64":      true,
-	"rune":       true,
-	"string":     true,
-	"uint":       true,
-	"uint8":      true,
-	"uint16":     true,
-	"uint32":     true,
-	"uint64":     true,
-	"uintptr":    true,
-	"true":       true,
-	"false":      true,
-	"iota":       true,
-	"nil":        true,
-	"append":     true,
-	"cap":        true,
-	"close":      true,
-	"complex":    true,
-	"copy":       true,
-	"delete":     true,
-	"imag":       true,
-	"len":        true,
-	"make":       true,
-	"new":        true,
-	"panic":      true,
-	"print":      true,
-	"println":    true,
-	"real":       true,
-	"recover":    true,
-}