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

[Arthur Eubanks via llvm-commits]

aeubanks wrote:

hmm yeah it does seem that `MCContext` is hard to unentangle from `MMI`/`MachineFunction`

is it possible to split out the "MMIWP doesn't own MMI" change? that seems less controversial and easier to review/merge and makes the `MCContext` changes easier to look at

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