[clang] [Clang] Detects invalid unicode characters in token formed by `##` (PR #208757)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 90316 tests passed
* 1720 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### Clang
<details>
<summary>Clang.Preprocessor/ucn-pp-identifier.c</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang -cc1 -internal-isystem /home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/23/include -nostdsysteminc /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Preprocessor/ucn-pp-identifier.c -fsyntax-only -std=c99 -pedantic -verify=expected,ext -Wundef -DTRIGRAPHS=1
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/bin/clang -cc1 -internal-isystem /home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/23/include -nostdsysteminc /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Preprocessor/ucn-pp-identifier.c -fsyntax-only -std=c99 -pedantic -verify=expected,ext -Wundef -DTRIGRAPHS=1
# .---command stderr------------
# | error: diagnostics with 'error' severity expected but not seen: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Preprocessor/ucn-pp-identifier.c Line 176 (directive at /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Preprocessor/ucn-pp-identifier.c:177) 'expected-error': expected unqualified-id
# | error: diagnostics with 'error' severity seen but not expected: 
# |   Line 176: expected identifier or '('
# | error: diagnostics with 'warning' severity seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Preprocessor/ucn-pp-identifier.c Line 172: 'CONCAT' macro redefined
# | error: diagnostics with 'note' severity seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/Preprocessor/ucn-pp-identifier.c Line 138: previous definition is here
# | 4 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/208757