[clang] [X86] Add ABI handling for __float128 (PR #75156)

[Phoebe Wang via cfe-commits]

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+// RUN: %clang_cc1 -triple x86_64-linux -emit-llvm  -target-feature +sse2 < %s | FileCheck %s --check-prefixes=CHECK
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phoebewang wrote:

I decided not to report error for `-sse` after investigating the current diagnosis machinism.
We didn't report the SSE error in semacheck but in backend due to backend crash.
`__float128` doesn't have crash issue and can be passed on GPR registers without SSE enabled.
IIUC, we prefer to be compatible with early version rather than always to GCC. So I'd like to keep the convention as is.

https://github.com/llvm/llvm-project/pull/75156