This makes the difference between errors and lower-level diagnostics
visible to users, and aligns the toolchain's behavior with the
expectations in `driver_fuzzer.cpp`.
Rearranges driver logic into CompilationUnits in order to associate
artifacts from the various stages of compilation.
Note, I'm not totally sure what the right thing to do is for
lower/codegen, so I'm just doing a rote change there for now that
mirrors prior phases (this is all the code supports anyways, so is
probably right for now regardless).
SourceBuffer error output is moved local for consistency with other
steps, and so that it's less ambiguous whether the error should be
expected to already include a filename.
Geoff Romer
Jon Ross-Perkins