[clang] [Clang][ARM][AArch64] Add branch protection attributes to the defaults. (PR #83277)

[Fangrui Song via cfe-commits]

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+// RUN: %clang_cc1 -triple aarch64-none-none -mbranch-target-enforce -msign-return-address=all -fcxx-exceptions -fexceptions -emit-llvm %s -o - | FileCheck %s --check-prefixes=CHECK
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MaskRay wrote:

perhaps it's time to add CodeGenCXX/AArch64/ ? We have more target-specific directories under CodeGen.

Then the weird `arm64-` prefix (not recommended for ELF/Arm official) can be removed.

You can remove `-fcxx-exceptions -fexceptions` since they are unused`. `-triple aarch64-none-none` can just be `-triple aarch64`.

https://github.com/llvm/llvm-project/pull/83277