libgomp.c++/target-std__multimap-concurrent.C: Fix USM memory freeing

Fix the unified-shared memory test,
   libgomp.c++/target-std__multimap-concurrent-usm.C
added in commit r16-1010-g83ca283853f195
   libgomp: Add testcases for concurrent access to standard C++ containers
   on offload targets, a number of USM variants
This tests includes the actual code of target-std__multimap-concurrent.C.

The issue is that multimap.insert allocates memory – which is freed by
the destructor. However, if the memory is allocated on a device
('insert'), it also needs to be freed there ('clear') as in general
freeing device-allocated memory is not possible on the host.

libgomp/ChangeLog:

	* testsuite/libgomp.c++/target-std__multimap-concurrent.C: Fix memory
	freeing of device allocated memory with USM.

Author: tob2

libgomp/testsuite/libgomp.c++/target-std__multimap-concurrent.C

@@ -4,6 +4,7 @@
 #include <stdlib.h>
 #include <time.h>
 #include <map>
+#include <omp.h>
 
 // Make sure that KEY_MAX is less than N to ensure some duplicate keys.
 #define N 3000
@@ -53,6 +54,16 @@ int main (void)
 	for (auto it = range.first; it != range.second; ++it)
 	  sum += (long long) it->first * it->second;
       }
+#ifdef MEM_SHARED
+  /* Even with USM, memory allocated on the device (with _map.insert)
+     must be freed on the device.  */
+  if (omp_get_default_device () != omp_initial_device
+      && omp_get_default_device () != omp_get_num_devices ())
+    {
+      #pragma omp target
+	_map.clear ();
+    }
+#endif
 
 #ifndef MEM_SHARED
   #pragma omp target