[libcxx] Fix __compressed_pair so it doesn't copy the argument multiple times, and add constexpr.

Summary:
__compressed_pair takes and passes it's constructor arguments by value. This causes arguments to be moved 3 times instead of once. This patch addresses that issue and fixes `constexpr` on the constructors.

I would rather have this fix than D27564, and I'm fairly confident it's not ABI breaking but I'm not 100% sure.

I prefer this solution because it removes a lot of code and makes the implementation *much* smaller.

Reviewers: mclow.lists, K-ballo

Reviewed By: K-ballo

Subscribers: K-ballo, cfe-commits

Differential Revision: https://reviews.llvm.org/D27565

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300140 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/memory/unique.ptr/unique.ptr.runtime/unique.ptr.runtime.ctor/default01.fail.cpp

@@ -15,24 +15,25 @@
 
 // default unique_ptr ctor should require default Deleter ctor
 
-
 #include <memory>
+#include "test_macros.h"
 
-class Deleter
-{
-    // expected-error@memory:* {{base class 'Deleter' has private default constructor}}
-    // expected-note@memory:* + {{in instantiation of member function}}
-    Deleter() {} // expected-note {{implicitly declared private here}}
+class Deleter {
+  Deleter() {}
 
 public:
+  Deleter(Deleter&) {}
+  Deleter& operator=(Deleter&) { return *this; }
 
-    Deleter(Deleter&) {}
-    Deleter& operator=(Deleter&) { return *this; }
-
-    void operator()(void*) const {}
+  void operator()(void*) const {}
 };
 
-int main()
-{
-    std::unique_ptr<int[], Deleter> p;
+int main() {
+#if TEST_STD_VER >= 11
+  // expected-error@memory:* {{call to implicitly-deleted default constructor}}
+  // expected-note@memory:* {{implicitly deleted because base class 'Deleter' has an inaccessible default constructor}}
+#else
+  // expected-error@memory:* {{base class 'Deleter' has private default constructor}}
+#endif
+  std::unique_ptr<int[], Deleter> p; // expected-note {{requested here}}
 }