The driver now looks for all files under core/prelude/ and considers
them all to be part of the prelude. The driver also now only processes
the prelude in `--phase=check` and later, when it would actually be
imported.
With that done, add a simple `carbon_binary` build rule and use it to
build the example in `//examples`. This should cause the example to be
built as part of our continuous integration.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
