Adding x86 support for to_ascii_port_clzll() (#1567)

Summary:
Currently to_ascii_port_clzll() in ToAscii.h always returns 0 in x86. This switches to __builtin_clzll() backed by a polyfill.

Closes https://github.com/facebook/folly/issues/1566.

Pull Request resolved: https://github.com/facebook/folly/pull/1567

Reviewed By: luciang

Differential Revision: D28162903

Pulled By: yfeldblum

fbshipit-source-id: 1872cda7aa155b84086bd15ad6cd40475eeb0ac4

folly/lang/ToAscii.h

@@ -24,10 +24,7 @@
 #include <folly/Utility.h>
 #include <folly/lang/Align.h>
 #include <folly/lang/CArray.h>
-
-#if _MSC_VER
-#include <intrin.h>
-#endif
+#include <folly/portability/Builtins.h>
 
 namespace folly {
 
@@ -56,18 +53,6 @@ using to_ascii_alphabet_upper = to_ascii_alphabet<true>;
 
 namespace detail {
 
-FOLLY_ERASE auto to_ascii_port_clzll(uint64_t v) {
-#if _MSC_VER
-#if FOLLY_X64
-  return __lzcnt64(v);
-#else
-  return __assume(0), 0;
-#endif
-#else
-  return __builtin_clzll(v);
-#endif
-}
-
 template <uint64_t Base, typename Alphabet>
 struct to_ascii_array {
   using data_type_ = c_array<uint8_t, Base>;
@@ -224,7 +209,7 @@ FOLLY_ALWAYS_INLINE size_t to_ascii_size_clzll(uint64_t v) {
   }
 
   //  log2 is approx log<2>(v)
-  size_t const vlog2 = 64 - static_cast<size_t>(to_ascii_port_clzll(v));
+  size_t const vlog2 = 64 - static_cast<size_t>(__builtin_clzll(v));
 
   //  handle directly when Base is power-of-two
   if (!(Base & (Base - 1))) {