[clang] [X86] Add F16C f16 -> f32 constexpr support (PR #158142)

[Simon Pilgrim via cfe-commits]

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@@ -20,6 +20,14 @@
 #define __DEFAULT_FN_ATTRS256 \
   __attribute__((__always_inline__, __nodebug__, __target__("f16c"), __min_vector_width__(256)))
 
+#ifdef __cplusplus
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RKSimon wrote:

Use this instead: `#if defined(__cplusplus) && (__cplusplus >= 201103L)`

https://github.com/llvm/llvm-project/pull/158142