Add missing builtin_test_helpers header

Author: markbhasawut

clang/test/CodeGen/X86/avx512fp16-builtins.c

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -ffreestanding -flax-vector-conversions=none %s -triple=x86_64-unknown-unknown -target-feature +avx512fp16 -emit-llvm -o - -Wall -Werror | FileCheck %s
 
 #include <immintrin.h>
+#include "builtin_test_helpers.h"
 
 _Float16 test_mm512_cvtsh_h(__m512h __A) {
   // CHECK-LABEL: @test_mm512_cvtsh_h