Mark test as XFAIL that started failing after 418f0066eb. (#85027)

Similar failures were previously seen and XFAILed in
https://reviews.llvm.org/D118468.

See the phabricator review for a description of the problem, and the
linked discourse thread for what the failing output looks like.

This change should fix the issue on two buildbots that are running older
versions of GDB:
- https://lab.llvm.org/buildbot/#/builders/217/builds/37559
- https://lab.llvm.org/buildbot/#/builders/247/builds/15173

cross-project-tests/debuginfo-tests/llgdb-tests/forward-declare-class.cpp

@@ -1,6 +1,7 @@
 // RUN: %clangxx %target_itanium_abi_host_triple -O0 -g %s -c -o %t.o
 // RUN: %test_debuginfo %s %t.o
 // Radar 9168773
+// XFAIL: !system-darwin && gdb-clang-incompatibility
 
 // DEBUGGER: ptype A
 // Work around a gdb bug where it believes that a class is a