Don't attempt to leverage the _cvtss_sh and _cvtsh_ss instructions when compiling with clang since it's unclear which versions of clang actually support these instruction.

Eigen/Core

diff --git a/Eigen/Core b/Eigen/Core
index ac58f10..5004013 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -204,7 +204,7 @@
   #endif
 #endif
 
-#if defined(__F16C__)
+#if defined(__F16C__) && !defined(EIGEN_COMP_CLANG)
   // We can use the optimized fp16 to float and float to fp16 conversion routines
   #define EIGEN_HAS_FP16_C
 #endif