Allow coroutine_handle<const T> to support creation from const references to the promise_type

It seems conceivable that a user would need to get a coroutine handle
having only a const reference to the promise_type, for example from
within a const member function of the promise.

This patch allows that use case. A coroutine_handle<const T> can be used
in essentially the same way a coroutine_handle<T>, ie to start and destroy
the coroutine. The constness of the promise doesn't/shouldn't propagate
to the handle.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305536 91177308-0d34-0410-b5e6-96231b3b80d8

include/experimental/coroutine

@@ -250,9 +250,11 @@ public:
 
     _LIBCPP_ALWAYS_INLINE
     static coroutine_handle from_promise(_Promise& __promise) _NOEXCEPT {
+        typedef typename remove_cv<_Promise>::type _RawPromise;
         coroutine_handle __tmp;
-        __tmp.__handle_ = __builtin_coro_promise(_VSTD::addressof(__promise),
-                                                 __alignof(_Promise), true);
+        __tmp.__handle_ = __builtin_coro_promise(
+            _VSTD::addressof(const_cast<_RawPromise&>(__promise)),
+             __alignof(_Promise), true);
         return __tmp;
     }
 };