[ffigen] Tighten up synthetic USRs

[liamappelbe]

USRs are unique IDs for declarations that are generated by clang. But in a few places we have to append things to them, or construct synthetic USRs. This was being done in an ad-hoc way, so I'm making it safer:

• Define a character that will never appear in real USRs from clang
• Assert that every real USR doesn't contain this character
• Switch each synthetic USR to include this character
  • ObjC blocks
  • Variadic function variants
  • Anonymous records

[github-actions]

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[coveralls]

coverage: 82.244% (-0.06%) from 82.303%
when pulling 6a2c865 on ffigen_cleanup_3
into d754fff on main.