[clang] [clang] Bind the __builtin_bit_cast operand as std::bit_cast binds its parameter (PR #231830)

[Eli Friedman via cfe-commits]

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+// RUN: %clang_cc1 -std=c++20 -fenable-matrix -O -emit-llvm -disable-llvm-passes -triple x86_64-unknown-linux-gnu %s -o - | FileCheck %s
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efriedma-quic wrote:

We don't have very much test coverage for it, but we do accept __builtin_bit_cast in C.  Which I'm not sure was a great idea, given the semantics are sort of tied to C++ type system rules, but it's there.

We should probably have some coverage for that.

https://github.com/llvm/llvm-project/pull/231830