[libc++] Extension: Make `move` and `forward` constexpr in C++11.

Summary:
`std::move` and `std::forward` were not marked constexpr in C++11.  This can be very damaging because it makes otherwise constant expressions non-constant. For example:

```
#include <utility>
template <class T>
struct Foo {
  constexpr Foo(T&& tx) :  t(std::move(tx)) {}
  T t;
};
[[clang::require_constant_initialization]] Foo<int> f(42); // Foo should be constant initialized but C++11 move is not constexpr. As a result `f` is an unsafe global.
```

This patch applies `constexpr` to `move` and `forward` as an extension in C++11. Normally the library is not allowed to add `constexpr` because it may be observable to the user. In particular adding constexpr may cause valid code to stop compiling. However these problems only happen in more complex situations, like making `__invoke(...)` constexpr. `forward` and `move` are simply enough that applying `constexpr` is safe. 

Note that libstdc++ has offered this extension since at least 4.8.1.

Most of the changes in this patch are simply test cleanups or additions. The main changes in the tests are:

* Fold all `forward_N.fail.cpp` tests into a single `forward.fail.cpp` test using -verify.
* Delete most `move_only_N.fail.cpp` tests because they weren't actually testing anything.
* Fold `move_copy.pass.cpp` and `move_only.pass.cpp` into a single `move.pass.cpp` test.
* Add return type and noexcept tests for `forward` and `move`.




Reviewers: rsmith, mclow.lists, EricWF

Subscribers: K-ballo, loladiro

Differential Revision: https://reviews.llvm.org/D24637

llvm-svn: 282439

1 files changed, 0 insertions, 49 deletions

diff --git a/libcxx/test/std/utilities/utility/forward/move_only3.fail.cpp b/libcxx/test/std/utilities/utility/forward/move_only3.fail.cpp
deleted file mode 100644
index 84c83ae48f8..00000000000
--- a/libcxx/test/std/utilities/utility/forward/move_only3.fail.cpp
+++ /dev/null
@@ -1,49 +0,0 @@
-//===----------------------------------------------------------------------===//
-//
-//                     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.
-//
-//===----------------------------------------------------------------------===//
-
-// test move
-
-#include <utility>
-#include <cassert>
-
-class move_only
-{
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    move_only(const move_only&);
-    move_only& operator=(const move_only&);
-#else  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    move_only(move_only&);
-    move_only& operator=(move_only&);
-#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
-
-public:
-
-#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    move_only(move_only&&) {}
-    move_only& operator=(move_only&&) {}
-#else  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
-    operator std::__rv<move_only> () {return std::__rv<move_only>(*this);}
-    move_only(std::__rv<move_only>) {}
-#endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
-
-    move_only() {}
-};
-
-move_only source() {return move_only();}
-const move_only csource() {return move_only();}
-
-void test(move_only) {}
-
-int main()
-{
-    move_only a;
-    const move_only ca = move_only();
-
-    test(std::move(ca));
-}