[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-win` running on `sie-win-worker` while building `clang` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/16524

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

```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'Clang :: OpenMP/teams_distribute_dist_schedule_messages.cpp' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
z:\b\llvm-clang-x86_64-sie-win\build\bin\clang.exe -cc1 -internal-isystem Z:\b\llvm-clang-x86_64-sie-win\build\lib\clang\21\include -nostdsysteminc -verify -fopenmp Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\OpenMP\teams_distribute_dist_schedule_messages.cpp -Wuninitialized
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\clang.exe' -cc1 -internal-isystem 'Z:\b\llvm-clang-x86_64-sie-win\build\lib\clang\21\include' -nostdsysteminc -verify -fopenmp 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\OpenMP\teams_distribute_dist_schedule_messages.cpp' -Wuninitialized
# .---command stderr------------
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\OpenMP\teams_distribute_dist_schedule_messages.cpp Line 111: cannot find start ('{{') of expected string
# |   File Z:\b\llvm-clang-x86_64-sie-win\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.
# `-----------------------------
# error: command failed with exit status: 1

--

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


```

</details>

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