[libc++][pstl][NFC] Remove usage of std::result_of from Parallel STL

std::result_of creates problems when building with C++20 because it's
deprecated there.

The solution is to remove it and get return value type for a function
with decltype.

Substitute std::invoke_result for std::result_of is unnecessary because
we don't have std::invoke semantics within the function - we don't work
with pointer-to-member's.

Reviewed by: ldionne, MikeDvorskiy, #libc

Differential Revision: https://reviews.llvm.org/D118457