[llvm] [Assignment Tracking] Change placeholder from `undef` to `poison` (PR #119760)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `lldb-x86_64-debian` running on `lldb-x86_64-debian` while building `llvm` at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/162/builds/12486

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test) failure: build (failure)
...
PASS: lldb-api :: tools/lldb-server/TestGdbRemoteRegisterState.py (146 of 2727)
PASS: lldb-api :: functionalities/data-formatter/data-formatter-stl/libcxx-simulators/string/TestDataFormatterLibcxxStringSimulator.py (147 of 2727)
PASS: lldb-api :: lang/cpp/namespace/TestNamespace.py (148 of 2727)
PASS: lldb-api :: commands/target/dump-separate-debug-info/dwo/TestDumpDwo.py (149 of 2727)
PASS: lldb-api :: lang/cpp/namespace_definitions/TestNamespaceDefinitions.py (150 of 2727)
PASS: lldb-api :: commands/command/script/TestCommandScript.py (151 of 2727)
PASS: lldb-api :: functionalities/tail_call_frames/ambiguous_tail_call_seq2/TestAmbiguousTailCallSeq2.py (152 of 2727)
PASS: lldb-api :: functionalities/tail_call_frames/disambiguate_call_site/TestDisambiguateCallSite.py (153 of 2727)
PASS: lldb-shell :: Subprocess/clone-follow-parent-wp.test (154 of 2727)
PASS: lldb-shell :: Subprocess/vfork-follow-child-wp.test (155 of 2727)
FAIL: lldb-api :: functionalities/thread/thread_specific_break_plus_condition/TestThreadSpecificBpPlusCondition.py (156 of 2727)
******************** TEST 'lldb-api :: functionalities/thread/thread_specific_break_plus_condition/TestThreadSpecificBpPlusCondition.py' FAILED ********************
Script:
--
/usr/bin/python3 /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/lldb/test/API/dotest.py -u CXXFLAGS -u CFLAGS --env LLVM_LIBS_DIR=/home/worker/2.0.1/lldb-x86_64-debian/build/./lib --env LLVM_INCLUDE_DIR=/home/worker/2.0.1/lldb-x86_64-debian/build/include --env LLVM_TOOLS_DIR=/home/worker/2.0.1/lldb-x86_64-debian/build/./bin --arch x86_64 --build-dir /home/worker/2.0.1/lldb-x86_64-debian/build/lldb-test-build.noindex --lldb-module-cache-dir /home/worker/2.0.1/lldb-x86_64-debian/build/lldb-test-build.noindex/module-cache-lldb/lldb-api --clang-module-cache-dir /home/worker/2.0.1/lldb-x86_64-debian/build/lldb-test-build.noindex/module-cache-clang/lldb-api --executable /home/worker/2.0.1/lldb-x86_64-debian/build/./bin/lldb --compiler /home/worker/2.0.1/lldb-x86_64-debian/build/./bin/clang --dsymutil /home/worker/2.0.1/lldb-x86_64-debian/build/./bin/dsymutil --make /usr/bin/make --llvm-tools-dir /home/worker/2.0.1/lldb-x86_64-debian/build/./bin --lldb-obj-root /home/worker/2.0.1/lldb-x86_64-debian/build/tools/lldb --lldb-libs-dir /home/worker/2.0.1/lldb-x86_64-debian/build/./lib -t /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/lldb/test/API/functionalities/thread/thread_specific_break_plus_condition -p TestThreadSpecificBpPlusCondition.py
--
Exit Code: 1

Command Output (stdout):
--
lldb version 20.0.0git (https://github.com/llvm/llvm-project.git revision 8820de68ddf02fe3c73def49ec32bbeca54c2754)
  clang revision 8820de68ddf02fe3c73def49ec32bbeca54c2754
  llvm revision 8820de68ddf02fe3c73def49ec32bbeca54c2754
Skipping the following test categories: ['libc++', 'dsym', 'gmodules', 'debugserver', 'objc']

--
Command Output (stderr):
--
Change dir to: /home/worker/2.0.1/lldb-x86_64-debian/llvm-project/lldb/test/API/functionalities/thread/thread_specific_break_plus_condition
UNSUPPORTED: LLDB (/home/worker/2.0.1/lldb-x86_64-debian/build/bin/clang-x86_64) :: test_python_dsym (TestThreadSpecificBpPlusCondition.ThreadSpecificBreakPlusConditionTestCase.test_python_dsym) (test case does not fall in any category of interest for this run) 
runCmd: settings clear -all

output: 

runCmd: settings set symbols.enable-external-lookup false

output: 

runCmd: settings set target.inherit-tcc true

output: 

runCmd: settings set target.disable-aslr false

output: 

runCmd: settings set target.detach-on-error false

output: 

```

</details>

https://github.com/llvm/llvm-project/pull/119760