all: remove use of deprecated NewMessage

Replace NewMessage calls with NewField, NewElement, or NewValue.

Change-Id: I6d2bb4f11f0eb2ba7a52308b1addb111137ad4b9
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193266
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/proto/decode.go b/proto/decode.go
index e5ecb3b..8974139 100644
--- a/proto/decode.go
+++ b/proto/decode.go
@@ -154,7 +154,7 @@
 	)
 	switch valField.Kind() {
 	case protoreflect.GroupKind, protoreflect.MessageKind:
-		val = protoreflect.ValueOf(mapv.NewMessage())
+		val = mapv.NewValue()
 	}
 	// Map entries are represented as a two-element message with fields
 	// containing the key and value.