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

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.