[clang] [llvm] [mlir] [IR] Do not store Function inside BlockAddress (PR #137958)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I think at some point we were discussing changing what happens when you have a dead blockaddress, which might involve a kind of blockaddress that doesn't refer to a specific basic block.  (The value of "1" has bad interactions if you compute the difference between two blockaddresses.)  But we can cross that bridge when we get there, I guess.

Otherwise looks fine.

https://github.com/llvm/llvm-project/pull/137958