Guard _Float16 use on clang >= 15

Change-Id: I2034b537182c42398659bbb1cbf56d7fdc3f7785
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/1041596
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

src/Transform_inl.h

diff --git a/src/Transform_inl.h b/src/Transform_inl.h
index aad0575..c0b2765 100644
--- a/src/Transform_inl.h
+++ b/src/Transform_inl.h
@@ -151,7 +151,7 @@
 #elif defined(USING_AVX512F)
     return (F)_mm512_cvtph_ps((__m256i)half);
 #elif defined(USING_AVX_F16C)
-#if defined(__clang__)
+#if defined(__clang__) && __clang_major__ >= 15 // for _Float16 support
     typedef _Float16 __attribute__((vector_size(16))) F16;
     return __builtin_convertvector((F16)half, F);
 #else