[clang] [clang][rtsan] Introduce realtime sanitizer codegen and driver (PR #102622)

[Fangrui Song via cfe-commits]

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+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -fsanitize=realtime -emit-llvm -o - %s | FileCheck %s
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MaskRay wrote:

this should be merged with `at_entry.c`.

https://github.com/llvm/llvm-project/pull/102622