Fix DensetSet::insert_as() for MSVC2015 (NFC)

[mehdi_amini]

Summary

The latest MSVC update apparently resolve the call from the
const ref variant to itself, leading to an infinite
recursion. It is not clear to me why the r-value overload is
not selected. ValueT is a pointer type, and the functional-style
cast in the call insert_as(ValueT(V), LookupKey); should result
in a r-value ref.

If anyone understand...

[chandlerc]

Provided this fixes the issue users are seeing with certain host compilers, it seems a good approach.

Not sure if you want to leave a comment about intentionally avoiding calling from one overload to the other?

Either way, feel free to submit if this helps folks.

llvm/trunk/include/llvm/ADT/DenseSet.h

std::pair<iterator, bool> insert(ValueT &&V) {
     return TheMap.try_emplace(std::move(V), Empty);
   }
 
   /// Alternative version of insert that uses a different (and possibly less
   /// expensive) key type.
   template <typename LookupKeyT>
   std::pair<iterator, bool> insert_as(const ValueT &V,
                                       const LookupKeyT &LookupKey) {
-    return insert_as(ValueT(V), LookupKey);
+    return TheMap.insert_as({V, detail::DenseSetEmpty()}, LookupKey);
   }
   template <typename LookupKeyT>
   std::pair<iterator, bool> insert_as(ValueT &&V, const LookupKeyT &LookupKey) {
-    detail::DenseSetEmpty Empty;
-    return TheMap.insert_as(std::make_pair(std::move(V), Empty), LookupKey);
+    return TheMap.insert_as({std::move(V), detail::DenseSetEmpty()}, LookupKey);
   }
 
   // Range insertion of values.
   template<typename InputIt>
   void insert(InputIt I, InputIt E) {
     for (; I != E; ++I)
       insert(*I);
   }
 };
 
 } // end namespace llvm
 
 #endif