avoid forcing the type of clzll

Summary: Between gnu and msvc compilers, the builtins and intrinsics `__builtin_clzll` and `__lzcnt64` have different return types. They are wrapped in a portability function in `to_ascii` which currently forces a return type, which return type could trip implicit-conversion warnings. Avoid forcing the return type to avoid tripping implicit-conversion warnings.

Reviewed By: luciang

Differential Revision: D27535348

fbshipit-source-id: 3339d5d04e095f936c18d2b39ac1fb4b32f8a810

folly/lang/ToAscii.h

@@ -56,7 +56,7 @@ using to_ascii_alphabet_upper = to_ascii_alphabet<true>;
 
 namespace detail {
 
-FOLLY_ERASE int to_ascii_port_clzll(uint64_t v) {
+FOLLY_ERASE auto to_ascii_port_clzll(uint64_t v) {
 #if _MSC_VER
 #if FOLLY_X64
   return __lzcnt64(v);