[clang] Add Function Argument Demotion support (PR #170915)

[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
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andykaylor wrote:

Can you add LLVM and OGCG checks for this test?

https://github.com/llvm/llvm-project/pull/170915