Mark the long overload of digits_to as noexcept

Summary: Well, the explicit instantion for it anyways. The extern template declaration for it declares it as `noexcept`, so MSVC generates an error because the explicit instantiation is not also marked as `noexcept`

Reviewed By: yfeldblum, ericniebler

Differential Revision: D3744090

fbshipit-source-id: 4e756b2761c23a436097a6131b9b2ecd59faf4f9

folly/Conv.cpp

@@ -631,7 +631,7 @@ template Expected<unsigned int, ConversionCode> digits_to<unsigned int>(
 
 template Expected<long, ConversionCode> digits_to<long>(
     const char*,
-    const char*);
+    const char*) noexcept;
 template Expected<unsigned long, ConversionCode> digits_to<unsigned long>(
     const char*,
     const char*) noexcept;