Enable -Wunused-value in glean/PACKAGE +1

Summary:
This diff enables compilation warning flags for the directory in question. Further details are in [this workplace post](https://fb.workplace.com/permalink.php?story_fbid=pfbid02XaWNiCVk69r1ghfvDVpujB8Hr9Y61uDvNakxiZFa2jwiPHscVdEQwCBHrmWZSyMRl&id=100051201402394).

This is a low-risk diff. There are **no run-time effects** and the diff has already been observed to compile locally. **If the code compiles, it work; test errors are spurious.**

Differential Revision: D70282465

fbshipit-source-id: 418812764823c9581bd20351ccd012588eb888d0

Author: r-barnes

glean/lang/clang/tests/smoke-xlang/test.cpp

@@ -24,7 +24,7 @@ void thriftCaller() {
   auto client = facebook::servicerouter::cpp2::getClientFactory().getSRClientUnique<SomeService>("dummy.tier");
   client->sync_someFunction(response, request);
   response.struct_field_1().value();
-  SomeEnum::EnumValue0;
+  std::ignore = SomeEnum::EnumValue0;
   smoke_constants::SOME_CONST();
-  smoke_constants::SOME_CONST_;
+  std::ignore = smoke_constants::SOME_CONST_;
 }