[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-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/10186

<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 :: SemaSYCL/sycl-kernel-entry-point-attr-appertainment.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\20\include -nostdsysteminc -triple x86_64-linux-gnu -std=c++17 -fsyntax-only -fsycl-is-device -verify Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\SemaSYCL\sycl-kernel-entry-point-attr-appertainment.cpp
# 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\20\include' -nostdsysteminc -triple x86_64-linux-gnu -std=c++17 -fsyntax-only -fsycl-is-device -verify 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\SemaSYCL\sycl-kernel-entry-point-attr-appertainment.cpp'
# .---command stderr------------
# | error: 'expected-error' diagnostics expected but not seen: 
# |   File Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\SemaSYCL\sycl-kernel-entry-point-attr-appertainment.cpp Line 173 (directive at Z:\b\llvm-clang-x86_64-sie-win\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 Z:\b\llvm-clang-x86_64-sie-win\llvm-project\clang\test\SemaSYCL\sycl-kernel-entry-point-attr-appertainment.cpp Line 173: an attribute list cannot appear here
# | 2 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

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


```

</details>

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