[libc++][PSTL] Make the PSTL available by default under -fexperimental-library

This removes the need for a custom libc++ build to have a basic set of PSTL algorithms.

Reviewed By: ldionne, #libc

Spies: miyuki, libcxx-commits, arichardson

Differential Revision: https://reviews.llvm.org/D149624
diff --git a/libcxx/include/algorithm b/libcxx/include/algorithm
index 41ce675..469bf17 100644
--- a/libcxx/include/algorithm
+++ b/libcxx/include/algorithm
@@ -1788,6 +1788,10 @@
 #include <__algorithm/partition_point.h>
 #include <__algorithm/pop_heap.h>
 #include <__algorithm/prev_permutation.h>
+#include <__algorithm/pstl_any_all_none_of.h>
+#include <__algorithm/pstl_fill.h>
+#include <__algorithm/pstl_find.h>
+#include <__algorithm/pstl_for_each.h>
 #include <__algorithm/push_heap.h>
 #include <__algorithm/ranges_adjacent_find.h>
 #include <__algorithm/ranges_all_of.h>
@@ -1908,13 +1912,6 @@
 #include <__algorithm/unwrap_iter.h>
 #include <__algorithm/upper_bound.h>
 
-#ifdef _LIBCPP_HAS_PARALLEL_ALGORITHMS
-#  include <__algorithm/pstl_any_all_none_of.h>
-#  include <__algorithm/pstl_fill.h>
-#  include <__algorithm/pstl_find.h>
-#  include <__algorithm/pstl_for_each.h>
-#endif
-
 // standard-mandated includes
 
 // [algorithm.syn]