Force LLVM to preserve value names in the same situations where we em…

[rjmccall]

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…it them in the first place.

Fixes IRGen test failures that only arise in non-asserts builds that
we've apparently been working around in less principled ways for
the last few months.

[rjmccall]

This isn't going to be usefully tested by smoke tests because it only changes things for non-asserts builds, which we don't have any of in the bots.