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/text/value.go b/internal/encoding/text/value.go
index b092598..f98dbcb 100644
--- a/internal/encoding/text/value.go
+++ b/internal/encoding/text/value.go
@@ -192,7 +192,7 @@
 		}
 		// C++ accepts large positive hex numbers as negative values.
 		// This feature is here for proto1 backwards compatibility purposes.
-		if flags.Proto1Legacy && len(v.raw) > 1 && v.raw[0] == '0' && v.raw[1] == 'x' {
+		if flags.ProtoLegacy && len(v.raw) > 1 && v.raw[0] == '0' && v.raw[1] == 'x' {
 			if !b64 {
 				return int64(int32(n)), n <= math.MaxUint32
 			}