[NFC][DebugInfo] Maintain RemoveDIs flag when attributor creates functions (#79143)

We're using this flag (IsNewDbgInfoFormat) to detect the boundaries in
LLVM of what's treating debug-info as intrinsics (i.e. dbg.value), and
what's using DPValue objects (the non-intrinsic replacement). The
attributor tends to create new wrapper functions and doesn't insert them
into Modules in the usual way, thus we have to manually update that flag
to signal what debug-info mode it's using.

I've added some --try-experimental-debuginfo-iterators RUN lines to
tests that would otherwise crash because of this, so that they're
exercised by our new-debuginfo-iterators buildbot.

NB: there's an attributor test with a dbg.value in it, however
attributes re-order themselves in RemoveDIs mode for various reasons, so
we're going to address that in a different patch.