let to_ascii_port_clzll always return non-void

Summary: Required for msvc 32-bit builds - the result of `to_ascii_port_clzll` is used in an arithmetic expression.

Reviewed By: SarahDesouky, Orvid

Differential Revision: D27585004

fbshipit-source-id: b84b5b6d6fe417f3c6adafcaf826505275aa827c

folly/lang/ToAscii.h

@@ -61,7 +61,7 @@ FOLLY_ERASE auto to_ascii_port_clzll(uint64_t v) {
 #if FOLLY_X64
   return __lzcnt64(v);
 #else
-  __assume(0);
+  return __assume(0), 0;
 #endif
 #else
   return __builtin_clzll(v);