[libcxx] Blind attempt to fix harmless leak in nodiscard_extensions.pass.cpp test

libcxx-libcxxabi-x86_64-linux-ubuntu-asan complains about a leak here.

llvm-svn: 342814

libcxx/test/libcxx/diagnostics/nodiscard_extensions.pass.cpp

@@ -21,6 +21,7 @@
 
 int main() {
   {
-    std::get_temporary_buffer<int>(1);
+    const auto p = std::get_temporary_buffer<int>(1);
+    std::return_temporary_buffer(p.first);
   }
 }