go/doc: add IsPredeclared function

IsPredeclared allows simplifying src/golang.org/x/tools/godoc/linkify.go

Change-Id: I56b3223896f844630bc2e940255572d1682f0d06
Reviewed-on: https://go-review.googlesource.com/29870Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/doc/reader.go

@@ -809,6 +809,11 @@ func noteBodies(notes []*Note) []string {
 // ----------------------------------------------------------------------------
 // Predeclared identifiers
 
+// IsPredeclared reports whether s is a predeclared identifier.
+func IsPredeclared(s string) bool {
+	return predeclaredTypes[s] || predeclaredFuncs[s] || predeclaredConstants[s]
+}
+
 var predeclaredTypes = map[string]bool{
 	"bool":       true,
 	"byte":       true,