[clang] [CIR] Add framework for CIR to LLVM IR lowering (PR #124650)

[Nathan Lanza via cfe-commits]

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@@ -44,7 +44,7 @@ class CIRGenerator : public clang::ASTConsumer {
   const clang::CodeGenOptions &codeGenOpts;
 
 protected:
-  std::unique_ptr<mlir::MLIRContext> mlirContext;
+  std::shared_ptr<mlir::MLIRContext> mlirContext;
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lanza wrote:

> Could CIRGenerator own this and just share it with lowerFromCIRToLLVMIR/etc via reference? CIRGenerator seems to be the entry point into all of CIR, so it makes sense for me for it to 'own' the lifetime.

I think hoisting it to the `CIRGenConsumer` and just letting the `CIRGenerator` get a handle to it makes sense. 

Alternatively, the `CIRGenerator` is done at this point. We could have some sort of destruction of the `CIRGenerator` here and have it give up it's state contents to the `CIRGenConsumer`. cc @bcardosolopes 

https://github.com/llvm/llvm-project/pull/124650