[clang] [flang] [llvm] [mlir] Make MMIWP not have ownership over MMI + Make MMI Only Use an External MCContext (PR #105541)

[Matin Raayai via cfe-commits]

matinraayai wrote:

@aeubanks @arsenm after looking into this in more detail, I realized that the `getContext` method of `MMI` is heavily used in the `AsmPrinter` to create symbols. Also not having it makes it harder for the `MMI` to create machine functions using `getOrCreateMachineFunction`. 

Hence I don't think it's a good idea to remove the `MCContext` reference from `MMI`. 

https://github.com/llvm/llvm-project/pull/105541