all: rename tag and flags for legacy support

Rename build tag "proto1_legacy" -> "protolegacy"
to be consistent with the "protoreflect" tag.

Rename flag constant "Proto1Legacy" -> "ProtoLegacy" since
it covers more than simply proto1 legacy features.
For example, it covers alpha-features of proto3 that
were eventually removed from the final proto3 release.

Change-Id: I0f4fcbadd4b5a61c87645e2e5be11d187e59157c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189345
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/encoding/wire/wire.go b/internal/encoding/wire/wire.go
index 224ddf7..6c4fd5a 100644
--- a/internal/encoding/wire/wire.go
+++ b/internal/encoding/wire/wire.go
@@ -494,7 +494,7 @@
 // Other than overflow, this does not check for field number validity.
 func DecodeTag(x uint64) (Number, Type) {
 	// NOTE: MessageSet allows for larger field numbers than normal.
-	if flags.Proto1Legacy {
+	if flags.ProtoLegacy {
 		if x>>3 > uint64(math.MaxInt32) {
 			return -1, 0
 		}