[llvm] [NFC] Add comment to describe the intention use of newly added `avail-extern-gv-in-addrspace-to-local` (PR #144911)

[Shilei Tian via llvm-commits]

shiltian wrote:

I didn't add the assertion part suggested by @teresajohnson because that would need the test case to be amdgcn specific as well.

https://github.com/llvm/llvm-project/pull/144911