Fix fbvector::swap impl

Summary:
[Folly] Fix `fbvector::swap` impl to do proper ADL.

Instead of passing template parameters, cast the arguments. That way non-template `swap` overloads can also be found.

Reviewed By: Gownta

Differential Revision: D9026136

fbshipit-source-id: 29240fed1c90d3556c2476917a21cbe1949c9a3c

folly/FBVector.h

@@ -185,7 +185,7 @@ class fbvector {
   static void swap(Impl& a, Impl& b) {
     using std::swap;
     if (!usingStdAllocator::value) {
-      swap<Allocator>(a, b);
+      swap(static_cast<Allocator&>(a), static_cast<Allocator&>(b));
     }
     a.swapData(b);
   }