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

[Simon Pilgrim 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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RKSimon wrote:

Worth adding a non-SSE RUN?

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