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

[weiwei chen via llvm-commits]

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@@ -124,10 +124,10 @@ class MachineModuleInfo {
   const LLVMTargetMachine &getTarget() const { return TM; }
 
   const MCContext &getContext() const {
-    return ExternalContext ? *ExternalContext : Context;
+    return ExternalContext ? *ExternalContext : *Context;
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weiweichen wrote:

> Having 2 modes here is a bad API

I personally agree with this in principal, but again, I don't have much context on why these two are here originally, so I'd prefer not having to decide the fate of this API while not understanding the original intension of the design. If anyone else has more context, I'd really love to hear and happy to make another PR to improve this. 

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