libstdc++: Fix nodiscard warnings in perf test for memory pools

The use of unnamed std::lock_guard temporaries was intentional here, as
they were used like barriers (but std::barrier isn't available until
C++20). But that gives nodiscard warnings, because unnamed temporary
locks are usually unintentional. Use named variables in new block scopes
instead.

libstdc++-v3/ChangeLog:

	* testsuite/performance/20_util/memory_resource/pools.cc: Fix
	-Wunused-value warnings about unnamed std::lock_guard objects.