[llvm] [Object][NFC] Add extern template declarations needed by llvm-objdump (PR #109156)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd approved this pull request.

This basically homes a canonical copy of the template types rather than relying on the ODR based emission.

https://github.com/llvm/llvm-project/pull/109156