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

[rarutyun]

Summary

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.

[MikeDvorskiy]

LGTM

pstl/include/pstl/internal/utils.h

 _PSTL_HIDE_FROM_ABI_PUSH
 
 namespace __pstl
 {
 namespace __internal
 {
 
 template <typename _Fp>
-typename std::result_of<_Fp()>::type
-__except_handler(_Fp __f)
+auto
+__except_handler(_Fp __f) -> decltype(__f())
 {
     try
     {
         return __f();
     }
     catch (const std::bad_alloc&)
     {
         throw; // re-throw bad_alloc according to the standard [algorithms.parallel.exceptions]