[libcxx] Fix PR 22468 - std::function<void()> does not accept
non-void-returning functions

Summary:
The bug can be found here: https://llvm.org/bugs/show_bug.cgi?id=22468

`__invoke_void_return_wrapper` is needed to properly handle calling a
function that returns a value but where the std::function return type
is void. Without this '-Wsystem-headers' will cause
`function::operator()(...)` to not compile.

Reviewers: eugenis, K-ballo, mclow.lists

Reviewed By: mclow.lists

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D7444