Use-list order assembly declarations for PR20515

I'm now looking at implementing `-preserve-ll-use-list-order` (PR20515)
to preserve use-list order when round-tripping to assembly -- the
counterpart to `-preserve-bc-use-list-order`.

After a quick look at `LLParser` and `AssemblyWriter`, I believe we can
use the same technique as for bitcode. In particular, we can predict in
`AssemblyWriter` how use-lists will look in the `LLParser` and write out
use-list-order directives to be processed as they're seen.

There are a couple of bike sheds to paint.

Use-list-order directives