[clang] [CIR] Add support for globals reference variables (PR #182608)

[Andy Kaylor via cfe-commits]

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+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir %s -o - | FileCheck %s --check-prefixes=CIR
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andykaylor wrote:

It might be worth adding checks before lowering prepare is run, to verify the ctor region is created where expected (as is done in `global-ctor-dtor.cpp`).

https://github.com/llvm/llvm-project/pull/182608