This patch updates the promise() member to match the current spec. Specifically it removes the non-const overload and make the return type of the const overload non-const. This patch also makes the ASSERT_NOT_NOEXCEPT tests libc++ specific, since other implementations may be free to strengthen the specification. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@303895 91177308-0d34-0410-b5e6-96231b3b80d8