fix std::out_of_range calls to use real constructors

Summary:
curiously, std::out_of_range doesn't have a zero-argument constructor according
to the spec. This was causing issues in my clang setup...

Test Plan: compiles.

Reviewed By: delong.j@fb.com

FB internal diff: D643363

folly/small_vector.h

@@ -846,14 +846,14 @@ public:
 
   reference at(size_type i) {
     if (i >= size()) {
-      throw std::out_of_range();
+      throw std::out_of_range("index out of range");
     }
     return (*this)[i];
   }
 
   const_reference at(size_type i) const {
     if (i >= size()) {
-      throw std::out_of_range();
+      throw std::out_of_range("index out of range");
     }
     return (*this)[i];
   }