[clang] [clang][NFC] simplify the unset check in `ParseLabeledStatement` (PR #117430)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-offload-libc-amdgpu-runtime` running on `omp-vega20-1` while building `clang` at step 10 "Add check check-offload".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/73/builds/9048

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

```
Step 10 (Add check check-offload) failure: 1200 seconds without output running [b'ninja', b'-j 32', b'check-offload'], attempting to kill
...
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug47654.cpp (947 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug49779.cpp (948 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug50022.cpp (949 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/test_libc.cpp (950 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/wtime.c (951 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu :: offloading/bug49021.cpp (952 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu :: offloading/std_complex_arithmetic.cpp (953 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/complex_reduction.cpp (954 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug49021.cpp (955 of 960)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/std_complex_arithmetic.cpp (956 of 960)
command timed out: 1200 seconds without output running [b'ninja', b'-j 32', b'check-offload'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1237.279616

```

</details>

https://github.com/llvm/llvm-project/pull/117430