Produce warnings for Chrome, resulting in errors.

Change-Id: Icda954e1d6c8da79ecf5eddbd542a38a8d97847a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/29670
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Ben Clayton <bclayton@google.com>