[DebugInfo][RemoveDIs] Convert back to intrinsic form for ThinLTO

As explained on discourse [0] (comment 12), to get the non-intrinsic form
of debug-info records enabled and testing, we're only using it inside of
the pass manager in LLVM right now. Things like the textual IR writer and
bitcode writing _passes_ are instrumented to convert back to
intrinsic-form when writing a module out, but it turns out we missed the
ThinLTO bitcode writing pass. That causes uh, all variable location
debug-info to be dropped in ThinLTO mode (oops).

This patch adds that conversion; it should be low risk as it's identical to
what happens in all the other passes. However should this commit turn out
to cause trouble, please instead revert d759618d or whichever is the
most recent commit to set UseNewDbgInfoFormat to default to true. That'll
revert LLVM back to the definitely-correct behaviour.

[0] https://discourse.llvm.org/t/rfc-instruction-api-changes-needed-to-eliminate-debug-intrinsics-from-ir/68939