Fix PR12999 - unordered_set::insert calls operator new when no insert occurs

Summary:
when `unordered_set::insert(value_type&&)` was called it would be treated like `unordered_set::emplace(Args&&)` and it would allocate and construct a node before trying to insert it.
This caused unnecessary allocations when the value was already in the set. This patch adds an overload to `__hash_table::__insert_unique` that specifically handles `value_type&&` more link `value_type const &`. 

This patch also adds a single unified insert function for values into  `__hash_table` called `__insert_unique_value` that handles the cases for `__insert_unique(value_type&&)` and `__insert_unique(value_type const &)`. 

This patch fixes PR12999: http://llvm.org/bugs/show_bug.cgi?id=12999.




Reviewers: mclow.lists, titus, danalbert

Reviewed By: danalbert

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D7570

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@239666 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/containers/unord/unord.set/insert_dup_alloc.pass.cpp

@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Check that we don't allocate when trying to insert a duplicate value into a
+// unordered_set. See PR12999 http://llvm.org/bugs/show_bug.cgi?id=12999
+
+#include <cassert>
+#include <unordered_set>
+#include "count_new.hpp"
+#include "MoveOnly.h"
+
+int main()
+{
+    {
+    std::unordered_set<int> s;
+    assert(globalMemCounter.checkNewCalledEq(0));
+
+    for(int i=0; i < 100; ++i)
+        s.insert(3);
+
+    assert(s.size() == 1);
+    assert(s.count(3) == 1);
+    assert(globalMemCounter.checkNewCalledEq(2));
+    }
+    assert(globalMemCounter.checkOutstandingNewEq(0));
+    globalMemCounter.reset();
+#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+    {
+    std::unordered_set<MoveOnly> s;
+    assert(globalMemCounter.checkNewCalledEq(0));
+
+    for(int i=0; i<100; i++)
+        s.insert(MoveOnly(3));
+
+    assert(s.size() == 1);
+    assert(s.count(MoveOnly(3)) == 1);
+    assert(globalMemCounter.checkNewCalledEq(2));
+    }
+    assert(globalMemCounter.checkOutstandingNewEq(0));
+    globalMemCounter.reset();
+#endif
+}