[llvm] [TargetMachine] Add `getFlatAddressSpace` to `TargetMachine` (PR #108594)

[Shilei Tian via llvm-commits]

shiltian wrote:

> In OldPM, there is an analysis to query. In New PM you construct the pass with the TM as an argument

That means we are gonna pull in the dependency `TargetPassConfig`. I didn't see any scalar optimization pass (or any other non-codegen pass) depending on it, and I'm not sure if it is appropriate to add that dependency. This "dependency" means the `LLVMCodeGen` component.

https://github.com/llvm/llvm-project/pull/108594