Always ask rustc for messages about artifacts, and always process them
Always ask rustc for messages about artifacts, and always process them

Rather than attempt to determine which compilations require metadata and
only ask rustc to emit messages about artifacts for those compilations,
just unconditionally generate and process such messages for all
compilations.

In addition to simplifying code, this also gives us that information for
timing purposes as well, even when not pipelining.

rust-lang/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,rust-lang/cargo