[clang] [OpenMP] Fix crash when diagnosing dist_schedule (PR #139277)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-ubuntu-fast` running on `sie-linux-worker` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/144/builds/24756

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: OpenMP/teams_distribute_dist_schedule_messages.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
/home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/21/include -nostdsysteminc -verify -fopenmp /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/teams_distribute_dist_schedule_messages.cpp -Wuninitialized # RUN: at line 1
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/bin/clang -cc1 -internal-isystem /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/build/lib/clang/21/include -nostdsysteminc -verify -fopenmp /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/teams_distribute_dist_schedule_messages.cpp -Wuninitialized
error: 'expected-error' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/teams_distribute_dist_schedule_messages.cpp Line 111: cannot find start ('{{') of expected string
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/OpenMP/teams_distribute_dist_schedule_messages.cpp Line 111: argument to 'dist_schedule' clause must be a strictly positive integer value
2 errors generated.

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/139277