[Allocator] Fix an obvious think-o with the move assignment

implementation of the SpecificBumpPtrAllocator -- we have to actually
move the subobject. =] Noticed when using this code more directly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206582 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Allocator.h

@@ -370,7 +370,7 @@ public:
   ~SpecificBumpPtrAllocator() { DestroyAll(); }
 
   SpecificBumpPtrAllocator &operator=(SpecificBumpPtrAllocator &&RHS) {
-    Allocator = RHS.Allocator;
+    Allocator = std::move(RHS.Allocator);
     return *this;
   }