Conv.h: fix a type error caught by clang

Summary: Need to be more explicit about types; it was trying to add types deduced to be `unsigned long` + `long` which didn't agree and could be an overflow.  Be explicit anyway about the types involved.  Appease Clang, and also make the code more obvious to the reader.

Test Plan: Tried with gcc 4.9, clang 3.5, clang 3.6.

Reviewed By: yfeldblum@fb.com

Subscribers: mathieubaudet, maoy, folly-diffs@, yzhan, yfeldblum, chalfant

FB internal diff: D2134814

Tasks: 7337462

Signature: t1:2134814:1433726862:5dd80b198187c610f793e26160919922863a22a2

Blame Revision: D1934777

folly/Conv.h

@@ -256,12 +256,11 @@ inline uint32_t digits10(uint64_t v) {
   // approximate log_10(v) == log_10(2) * bits.
   // Integer magic below: 77/256 is appx. 0.3010 (log_10(2)).
   // The +1 is to make this the ceiling of the log_10 estimate.
-  const auto minLength = 1 + ((bits * 77) >> 8);
+  const uint32_t minLength = 1 + ((bits * 77) >> 8);
 
   // return that log_10 lower bound, plus adjust if input >= 10^(that bound)
   // in case there's a small error and we misjudged length.
-  return minLength +
-         (UNLIKELY (v >= powersOf10[minLength]));
+  return minLength + (uint32_t) (UNLIKELY (v >= powersOf10[minLength]));
 
 #else