Adjust the constexpr strlen implementation to handle up to 4kb strings

Summary:
Previously it was only capable of handling 512 byte strings under MSVC due to the constexpr recursion depth limit being 512 by default under MSVC.
In the future once we have C++14 constexpr everywhere, this should just be a for loop, which should remove the limit.

Reviewed By: yfeldblum

Differential Revision: D9230717

fbshipit-source-id: 88bcf0ef94b200301eb41392482e910b2e8de008

folly/portability/Constexpr.h

@@ -28,7 +28,18 @@ namespace detail {
 
 template <typename Char>
 constexpr size_t constexpr_strlen_internal(const Char* s, size_t len) {
-  return *s == Char(0) ? len : constexpr_strlen_internal(s + 1, len + 1);
+  // clang-format off
+  return
+      *(s + 0) == Char(0) ? len + 0 :
+      *(s + 1) == Char(0) ? len + 1 :
+      *(s + 2) == Char(0) ? len + 2 :
+      *(s + 3) == Char(0) ? len + 3 :
+      *(s + 4) == Char(0) ? len + 4 :
+      *(s + 5) == Char(0) ? len + 5 :
+      *(s + 6) == Char(0) ? len + 6 :
+      *(s + 7) == Char(0) ? len + 7 :
+      constexpr_strlen_internal(s + 8, len + 8);
+  // clang-format on
 }
 static_assert(
     constexpr_strlen_internal("123456789", 0) == 9,