Use unreachable in nix::listNarImpl()

[iljah]

Prevent

../src/libutil/nar-accessor.cc: In function ”nix::ListNarResult<deep> nix::listNarImpl(SourceAccessor&, const CanonPath&) [with bool deep = true]”:
../src/libutil/nar-accessor.cc:335:1: varoitus: ei-void-tyyppisen funktion loppu saavutettu [-Wreturn-type]
  335 | }
      | ^
../src/libutil/nar-accessor.cc: In function ”nix::ListNarResult<deep> nix::listNarImpl(SourceAccessor&, const CanonPath&) [with bool deep = false]”:
../src/libutil/nar-accessor.cc:335:1: varoitus: ei-void-tyyppisen funktion loppu saavutettu [-Wreturn-type]

Motivation

Context

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[Ericson2314]

Call unreachable() because the switch is supposed to be exhaustive. The fact that only some compilers give this error and not others makes me wonder if we can fix it with compiler flags too

[iljah]

Thanks, switched to unreachable().

[xokdvium]

Generally, we prefer nix::unreachable() because it doesn't invoke UB and just fails at runtime unlike std::unreachable. It's also marked [[noreturn]], so it should silence the diagnostic.