config.mak.dev: enable -Wunreachable-code

Having the compiler point out unreachable code can help avoid bugs, like
the one discussed in:

  https://lore.kernel.org/git/20250307195057.GA3675279@coredump.intra.peff.net/

In that case it was found by Coverity, but finding it earlier saves
everybody time and effort.

We can use -Wunreachable-code to get some help from the compiler here.
Interestingly, this is a noop in gcc. It was a real warning up until gcc
4.x, when it was removed for being too flaky, but they left the
command-line option to avoid breaking users. See:

  https://stackoverflow.com/questions/17249934/why-does-gcc-not-warn-for-unreachable-code

However, clang does implement this option, and it finds the case
mentioned above (and no other cases within the code base). And since we
run clang in several of our CI jobs, that's enough to get an early
warning of breakage.

We could enable it only for clang, but since gcc is happy to ignore it,
it's simpler to just turn it on for all developer builds.

Signed-off-by: Jeff King <peff@peff.net>
[jc: squashed meson.build change sent by Patrick]
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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