[clang] [llvm] [openmp] [Clang][OpenMP] Implement Loop splitting `#pragma omp split` directive (PR #183261)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-s390x-linux` running on `systemz-1` while building `clang,llvm,openmp` at step 6 "test-openmp".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/88/builds/22037

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

```
Step 6 (test-openmp) failure: test (failure)
******************** TEST 'libomp :: transform/split/fill_first.c' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/./bin/clang -fopenmp   -I /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/src -I /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test -L /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/src  -fno-omit-frame-pointer -mbackchain -I /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test/ompt /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test/transform/split/fill_first.c -o /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/test/transform/split/Output/fill_first.c.tmp -lm -latomic && /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/test/transform/split/Output/fill_first.c.tmp | tee /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/test/transform/split/Output/fill_first.c.tmp.out | /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/./bin/FileCheck /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test/transform/split/fill_first.c --match-full-lines
# executed command: /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/./bin/clang -fopenmp -I /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/src -I /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test -L /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/src -fno-omit-frame-pointer -mbackchain -I /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test/ompt /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test/transform/split/fill_first.c -o /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.build/runtimes/runtimes-bins/openmp/runtime/test/transform/split/Output/fill_first.c.tmp -lm -latomic
# .---command stderr------------
# | /home/uweigand/sandbox/buildbot/openmp-s390x-linux/llvm.src/openmp/runtime/test/transform/split/fill_first.c:8:19: error: unexpected OpenMP clause 'counts' in directive '#pragma omp split'
# |     8 | #pragma omp split counts(omp_fill, 2)
# |       |                   ^
# | 1 error generated.
# `-----------------------------
# error: command failed with exit status: 1

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/183261