debug/dwarf: add Entry.AttrField method to get *Field by Attr

Currently, Entry has a Val method that looks up an attribute and
returns its value. Now that Field has more fields than the attribute
and its value, it's useful to return the whole Field and let the
caller retrieve the parts it needs.

This change adds an AttrField method to Entry that does the same
lookup at Val, but returns the whole *Field rather than just the
value.

Change-Id: Ic629744c14c0e09d7528fa1026b0e1857789948c
Reviewed-on: https://go-review.googlesource.com/8503
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/debug/dwarf/entry.go

@@ -335,9 +335,18 @@ func (i Class) GoString() string {
 //	v, ok := e.Val(AttrSibling).(int64)
 //
 func (e *Entry) Val(a Attr) interface{} {
-	for _, f := range e.Field {
+	if f := e.AttrField(a); f != nil {
+		return f.Val
+	}
+	return nil
+}
+
+// AttrField returns the Field associated with attribute Attr in
+// Entry, or nil if there is no such attribute.
+func (e *Entry) AttrField(a Attr) *Field {
+	for i, f := range e.Field {
 		if f.Attr == a {
-			return f.Val
+			return &e.Field[i]
 		}
 	}
 	return nil