title: Clang generates errors when I try to use AVX features
body: <p>I'm on Windows 10, with Clang version 5 (recently installed). When I compile the following</p>
  
  <pre><code>#define __AVX__
  #define __AVX2__
  #include &lt;immintrin.h&gt;
  
  int main(void) {
      __m256i a, b, result;
      result = _mm256_add_epi64(a, b);
      return 0;
  }
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code> error: always_inline function '_mm256_add_epi64' requires target 
   feature 'avx2', but would be inlined into function 'main' that is 
   compiled without support for 'avx2'
      result = _mm256_add_epi64(a, b);
               ^
  </code></pre>
  
  <p>MSVC compiles it just fine. What compiler option do I need to use in order to get Clang to compile this? Or is there something else going on?</p>