[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 sure I can split the PR into two parts:
1. Make MMIWP not have ownership over MMI.
2. Make MMI only use an external context.

I will make a new PR for each of them and close this one once both are merged.

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