internal/impl: fix SetLazyValue
internal/impl: fix SetLazyValue

Since the internal body calls ExtensionType.ValueOf, it seems that the
intent is for lazyExtensionValue.value to store a:
	func() protoreflect.Value
instead of a:
	func() interface{}

This seems more apparent given that GetValue returns a pref.Value.

Change-Id: I1679fe56088c20d5c8d36360e75dd773850da4c2
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193757
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go