[llvm] [TableGen][DecoderEmitter] Avoid using a sentinel value (PR #153986)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve2-vla` running on `linaro-g4-02` while building `llvm` at step 7 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/198/builds/7032

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

```
Step 7 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'lit :: googletest-timeout.py' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 9
not env -u FILECHECK_OPTS "/usr/bin/python3.10" /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout    --param gtest_filter=InfiniteLoopSubTest --timeout=1 > /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1/utils/lit/tests/Output/googletest-timeout.py.tmp.cmd.out
# executed command: not env -u FILECHECK_OPTS /usr/bin/python3.10 /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit.py -j1 --order=lexical -v Inputs/googletest-timeout --param gtest_filter=InfiniteLoopSubTest --timeout=1
# .---command stderr------------
# | lit.py: /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit/main.py:73: note: The test suite configuration requested an individual test timeout of 0 seconds but a timeout of 1 seconds was requested on the command line. Forcing timeout to be 1 seconds.
# | Traceback (most recent call last):
# |   File "/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit/formats/googletest.py", line 307, in post_process_shard_results
# |     testsuites = json.load(f)["testsuites"]
# |   File "/usr/lib/python3.10/json/__init__.py", line 293, in load
# |     return loads(fp.read(),
# |   File "/usr/lib/python3.10/json/__init__.py", line 346, in loads
# |     return _default_decoder.decode(s)
# |   File "/usr/lib/python3.10/json/decoder.py", line 337, in decode
# |     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
# |   File "/usr/lib/python3.10/json/decoder.py", line 355, in raw_decode
# |     raise JSONDecodeError("Expecting value", s, err.value) from None
# | json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
# | 
# | During handling of the above exception, another exception occurred:
# | 
# | Traceback (most recent call last):
# |   File "/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit.py", line 6, in <module>
# |     main()
# |   File "/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit/main.py", line 131, in main
# |     selected_tests, discovered_tests = GoogleTest.post_process_shard_results(
# |   File "/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/llvm/llvm/utils/lit/lit/formats/googletest.py", line 309, in post_process_shard_results
# |     raise RuntimeError(
# | RuntimeError: Failed to parse json file: /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1/utils/lit/tests/Inputs/googletest-timeout/DummySubDir/OneTest.py-googletest-timeout-3939833-1-2.json
# | 
# `-----------------------------
# RUN: at line 11
FileCheck --check-prefix=CHECK-INF < /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1/utils/lit/tests/Output/googletest-timeout.py.tmp.cmd.out /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1/utils/lit/tests/googletest-timeout.py
# executed command: FileCheck --check-prefix=CHECK-INF /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1/utils/lit/tests/googletest-timeout.py
# .---command stderr------------
# | /home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/stage1/utils/lit/tests/googletest-timeout.py:34:14: error: CHECK-INF: expected string not found in input
# | # CHECK-INF: Timed Out: 1
# |              ^
# | <stdin>:13:29: note: scanning from here
# | Reached timeout of 1 seconds
# |                             ^
# | <stdin>:15:21: note: possible intended match here
# | TIMEOUT: googletest-timeout :: DummySubDir/OneTest.py/1/2 (2 of 2)
# |                     ^
# | 
# | Input file: <stdin>
...

```

</details>

https://github.com/llvm/llvm-project/pull/153986