[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, 25 deletions

diff --git a/libcxx/test/std/utilities/utility/forward/forward4.fail.cpp b/libcxx/test/std/utilities/utility/forward/forward4.fail.cpp
deleted file mode 100644
index 276506f811b..00000000000
--- a/libcxx/test/std/utilities/utility/forward/forward4.fail.cpp
+++ /dev/null
@@ -1,25 +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 forward
-
-#include <utility>
-
-struct A
-{
-};
-
-A source() {return A();}
-const A csource() {return A();}
-
-int main()
-{
-    const A ca = A();
-    std::forward<A>(ca);  // error
-}