protogen: fix name mangling for fields with identical GoCamelCase

This turns the nameclash testprotos ending in 3 into .proto
files which use syntax = "proto3". Prior to the Opaque API release,
we changed these .proto files to use editions, but using editions
with proto3 semantics is not exactly the same as using proto3,
so we need these .proto files to use syntax = "proto3";

Fixes golang/protobuf#1675

Change-Id: I0ebb37d4e1bf0fc830bbe613148429ec6ff5c46d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/644437
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

cmd/protoc-gen-go/testdata/nameclash/test_name_clash_hybrid.proto

@@ -331,3 +331,9 @@ message M20 {
     int32 which_which_foo = 2;
   }
 }
+
+message M21 {
+  string _foo = 1;
+  string X_foo = 2;
+  string get_x_foo = 3;
+}