[libc++] Fix std::forward for r-value references

The explicit cast is necessary even though the value for `t` is passed in as an r-value, because it becomes an l-value as soon as it is assigned to the variable.
5 jobs for forward-rvalue-ref
Name Stage Failure
git-update Git Update There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job log