reflect: add method StructTag.Lookup

The Lookup method provides a way to extract a tag value, while
determining whether the tag key exists in the struct field's tag.

Fixes #14883

Change-Id: I7460cb68f0ca1aaa025935050b9e182efcb64db3
Reviewed-on: https://go-review.googlesource.com/20864
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/reflect/type.go

@@ -976,8 +976,20 @@ type StructTag string
 // Get returns the value associated with key in the tag string.
 // If there is no such key in the tag, Get returns the empty string.
 // If the tag does not have the conventional format, the value
-// returned by Get is unspecified.
+// returned by Get is unspecified. To determine whether a tag is
+// explicitly set to the empty string, use Lookup.
 func (tag StructTag) Get(key string) string {
+	v, _ := tag.Lookup(key)
+	return v
+}
+
+// Lookup returns the value associated with key in the tag string.
+// If the key is present in the tag the value (which may be empty)
+// is returned. Otherwise the returned value will be the empty string.
+// The ok return value reports whether the value was explicitly set in
+// the tag string. If the tag does not have the conventional format,
+// the value returned by Lookup is unspecified.
+func (tag StructTag) Lookup(key string) (value string, ok bool) {
 	// When modifying this code, also update the validateStructTag code
 	// in golang.org/x/tools/cmd/vet/structtag.go.
 
@@ -1025,10 +1037,10 @@ func (tag StructTag) Get(key string) string {
 			if err != nil {
 				break
 			}
-			return value
+			return value, true
 		}
 	}
-	return ""
+	return "", false
 }
 
 // Field returns the i'th struct field.