[clang] [SYCL] Basic diagnostics for the sycl_kernel_entry_point attribute. (PR #120327)

[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/15300

<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 :: SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
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/20/include -nostdsysteminc -triple x86_64-linux-gnu -std=c++17 -fsyntax-only -fsycl-is-device -verify /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp
+ /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/20/include -nostdsysteminc -triple x86_64-linux-gnu -std=c++17 -fsyntax-only -fsycl-is-device -verify /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp
error: 'expected-error' diagnostics expected but not seen: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp Line 173 (directive at /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp:172): 'sycl_kernel_entry_point' attribute only applies to functions
error: 'expected-error' diagnostics seen but not expected: 
  File /home/buildbot/buildbot-root/llvm-clang-x86_64-sie-ubuntu-fast/llvm-project/clang/test/SemaSYCL/sycl-kernel-entry-point-attr-appertainment.cpp Line 173: an attribute list cannot appear here
2 errors generated.

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/120327