build: adding -Wreturn-type (#15870)
build: adding -Wreturn-type (#15870)

Futureproofing the one-off in #15858

Risk Level: low
Testing: n/a
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Alyssa Wilk <b9253d1ec9cc3e9d2aab68d288ba3dab3452adf6@chromium.org>
envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy