[compiler-rt] [llvm] [clang-tools-extra] [clang] [InferAddressSpaces] Fix constant replace to avoid modifying other functions (PR #70611)

[Matt Arsenault via cfe-commits]

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

I think it would be better if we could eliminate ConstantExpr addrspacecasts from the IR altogether, which would avoid most of the complexity here. I would also somewhat prefer to push this DFS into a helper function, but can live with it inline as-is

https://github.com/llvm/llvm-project/pull/70611