[pstl] Fix incorrect usage of std::invoke_result

std::invoke_result takes function object type and arguments separately
(unlike std::result_of) so, std::invoke_result_t<F()> usage is
incorrect.

On the other hand, we don't need std::invoke() semantics here at all. So
just simplifying the code without extra dependency and use trailing
return type as the fix.

Reviewed By: MikeDvorskiy

Differential Revision: https://reviews.llvm.org/D114624