[Allocator] Constrain the Deallocate templated overloads to only apply

to types which we can compute the size of. The comparison with zero
isn't actually interesting here, it's mostly about putting sizeof into
a sfinae context.

This is particular important for Deallocate as otherwise the void*
overload can quickly become ambiguous.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206251 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Allocator.h

@@ -97,12 +97,15 @@ public:
   }
 
   /// \brief Deallocate space for one object without destroying it.
-  template <typename T> void Deallocate(T *Ptr) {
+  template <typename T>
+  typename std::enable_if<sizeof(T) != 0, void>::type Deallocate(T *Ptr) {
     Deallocate(static_cast<const void *>(Ptr));
   }
 
   /// \brief Allocate space for an array of objects without constructing them.
-  template <typename T> void Deallocate(T *Ptr, size_t /*Num*/) {
+  template <typename T>
+  typename std::enable_if<sizeof(T) != 0, void>::type
+  Deallocate(T *Ptr, size_t /*Num*/) {
     Deallocate(static_cast<const void *>(Ptr));
   }
 };