[llvm] LangRef: Clarify that unnamed_addr constants can be duplicated. (PR #199251)

[Peter Collingbourne via llvm-commits]

pcc wrote:

> The set of transforms we currently do is known not to be self-consistent. Declaring one set legal, and the other illegal, is not usefully clarifying anything.
> 
> If you want to say something here, it's probably better to just describe the current status: duplication causes miscompiles, but LTO does it anyway.

Okay, I added a warning to cover cases like #32127.

https://github.com/llvm/llvm-project/pull/199251