[clang] [Clang] fix bad codegen from constexpr structured bindings (PR #186594)

[Eli Friedman via cfe-commits]

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+// RUN: %clang -std=c++26 -O3 -emit-llvm -S -target x86_64-unknown-linux-gnu -o - %s | FileCheck %s
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efriedma-quic wrote:

Codegen tests should use clang_cc1.  (This is partially historical, but it also avoid accidentally depending on various bits of driver behavior.)

We try to avoid using -O3 in clang codegen tests so that changes to LLVM IR optimizations don't affect the tests.

https://github.com/llvm/llvm-project/pull/186594