This makes several small changes to how the platform and device info
queries are handled:
* ReturnHelper has been replaced with InfoWriter which is more explicit
in how it is invoked.
* InfoWriter consumes `llvm::Expected` rather than values directly, and
will early exit if it returns an error.
* As a result of the above, `GetInfoString` now correctly returns errors
rather than empty strings.
* The host device now has its own dedicated "getInfo" function rather
than being checked in multiple places.
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