[PATCH] fix breakage from "libstdc++: Remove unnecessary uses of <utility>"

[Hans-Peter Nilsson <hp@bitrange.com>]

Commit r12-2534 was incomplete and (by inspection derived from
an MMIX build) failing for targets without an insn for
compare_and_swap for pointer-size objects, IOW for targets for
which "ATOMIC_POINTER_LOCK_FREE != 2" is true:

x/gcc/libstdc++-v3/src/c++17/memory_resource.cc: In member function
 'std::pmr::memory_resource*
 std::pmr::{anonymous}::atomic_mem_res::exchange(std::pmr::memory_resource*)':
x/gcc/libstdc++-v3/src/c++17/memory_resource.cc:140:21: error:
 'exchange' is not a member of 'std'
  140 |         return std::exchange(val, r);
      |                     ^~~~~~~~
make[5]: *** [Makefile:577: memory_resource.lo] Error 1
make[5]: Leaving directory
 '/home/hp/tmp/newmmix-r12-2579-p3/gccobj/mmix/libstdc++-v3/src/c++17'

This fix was derived from edits elsewhere in that patch.

Tested mmix-knuth-mmixware, restoring build (together with
target-reviving patches as MMIX is currently and at that commit
broken for target-specific reasons).

Ok to commit?

libstdc++-v3/:
	* src/c++17/memory_resource.cc: Use __exchange instead
	of std::exchange.
---
 libstdc++-v3/src/c++17/memory_resource.cc | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libstdc++-v3/src/c++17/memory_resource.cc b/libstdc++-v3/src/c++17/memory_resource.cc
index 5dfc29fc0ec8..1ba79903f870 100644
--- a/libstdc++-v3/src/c++17/memory_resource.cc
+++ b/libstdc++-v3/src/c++17/memory_resource.cc
@@ -29,7 +29,7 @@
 #include <new>
 #if ATOMIC_POINTER_LOCK_FREE != 2
 # include <bits/std_mutex.h>	// std::mutex, std::lock_guard
-# include <bits/move.h>		// std::exchange
+# include <bits/move.h>		// std::__exchange
 #endif

 namespace std _GLIBCXX_VISIBILITY(default)
@@ -117,7 +117,7 @@ namespace pmr
       memory_resource* exchange(memory_resource* r)
       {
 	lock_guard<mutex> lock(mx);
-	return std::exchange(val, r);
+	return std::__exchange(val, r);
       }
     };
 #else
@@ -137,7 +137,7 @@ namespace pmr

       memory_resource* exchange(memory_resource* r)
       {
-	return std::exchange(val, r);
+	return std::__exchange(val, r);
       }
     };
 #endif // ATOMIC_POINTER_LOCK_FREE == 2
-- 
2.20.1