[llvm] [CodeGen] Fix `MachineModuleInfo`'s move constructor to be more safe with `MCContext` ownership. (PR #104834)

[weiwei chen via llvm-commits]

weiweichen wrote:

@hgreving2304, I think were the original author who puts the [`ExternContext`](https://github.com/llvm/llvm-project/blob/e56ad22b4a41e65984d6997b2c2496a20f906d1d/llvm/include/llvm/CodeGen/MachineModuleInfo.h#L92) in `llvm::MachineModuleInfo`. Would you mind giving us a bit more context on why it was added originally, and if there is any concerns on removing it with the change of make `MCContext` not owned by `llvm::MachineModuleInfo` entirely anymore?

https://github.com/llvm/llvm-project/pull/104834