[libc++][test] Adds a TODO.

This addresses the post-commit review comment in
llvm#100775

Author: mordante

libcxx/test/std/utilities/utility/mem.res/mem.poly.allocator.class/mem.poly.allocator.class.general/equality.pass.cpp

@@ -7,6 +7,7 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14
+// TODO: Change to XFAIL once https://github.com/llvm/llvm-project/issues/40340 is fixed
 // UNSUPPORTED: availability-pmr-missing
 
 // <memory_resource>