[llvm] Verify threadlocal_address constraints (PR #87841)

[Matthias Braun via llvm-commits]

MatzeB wrote:

It seems code like my #87844 is slightly simpler when it can rely on there only ever being a `GlobalVariable`. So I'll go an change the language reg.

That said: If this pattern is helpful to support then I can trivially add some loop to skip address space casts in #87844 instead, so this isn't really blocking anything. I was just wondering what the rules are...

https://github.com/llvm/llvm-project/pull/87841