proto: enable/disable fast path with build tags

Remove the Reflection field from MarshalOptions and UnmarshalOptions.
Disable the fast path and use the reflection-based implementation when
the 'protoreflect' build tag is set.

Change-Id: Ic674e3af67501de27fb03ec2712fbed40eae7fef
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/170896
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/integration_test.go b/integration_test.go
index f62e072..7dd6b24 100644
--- a/integration_test.go
+++ b/integration_test.go
@@ -66,6 +66,7 @@
 			runGo("Build", workDir, "go", "build", "./...")
 			runGo("TestNormal", workDir, "go", "test", "-race", "./...")
 			runGo("TestPureGo", workDir, "go", "test", "-race", "-tags", "purego", "./...")
+			runGo("TestReflect", workDir, "go", "test", "-race", "-tags", "protoreflect", "./...")
 			if v == golangLatest {
 				runGo("TestProto1Legacy", workDir, "go", "test", "-race", "-tags", "proto1_legacy", "./...")
 				runGo("TestProtocGenGo", "cmd/protoc-gen-go/testdata", "go", "test")