Parallel for_each iterators don't have to be mutable

The overloads of for_each that take an execution policy argument do not
require that the iterator arguments always be mutable iterators.  Whether
or not the iterators need to be mutable depends on what the function
object does.  for_each is unusual, if not unique, in this regard; for all
other algorithms the mutability of the iterators is specified by the
algorithm.  See http://eel.is/c++draft/alg.foreach#7 , though I admit that
the wording is not particularly clear about this.

Since the compiler can't realiably figure out whether or not the function
object modifies the elements in the sequence, for_each should only check
for constant iterators (_REQUIRE_PARALLEL_ITERATOR), not mutable iterators
(_REQUIRE_CPP17_MUTABLE_ITERATOR).

This bug was introduced by microsoft#2960 .
That PR should not have changed for_each.

stl/inc/execution

@@ -1189,7 +1189,7 @@ struct _Static_partitioned_for_each2 { // for_each task scheduled on the system
 template <class _ExPo, class _FwdIt, class _Fn, _Enable_if_execution_policy_t<_ExPo> /* = 0 */>
 void for_each(_ExPo&&, _FwdIt _First, _FwdIt _Last, _Fn _Func) noexcept /* terminates */ {
     // perform function for each element [_First, _Last) with the indicated execution policy
-    _REQUIRE_CPP17_MUTABLE_ITERATOR(_FwdIt);
+    _REQUIRE_PARALLEL_ITERATOR(_FwdIt);
     _Adl_verify_range(_First, _Last);
     auto _UFirst      = _Get_unwrapped(_First);
     const auto _ULast = _Get_unwrapped(_Last);