[clang] [Clang] Avoid crash when expression requirements are substitution failures (PR #176896)

[via cfe-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 51871 tests passed
* 908 tests skipped
* 1 test failed

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

### Clang
<details>
<summary>Clang.SemaTemplate/concepts.cpp</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
c:\_work\llvm-project\llvm-project\build\bin\clang.exe -cc1 -internal-isystem C:\_work\llvm-project\llvm-project\build\lib\clang\23\include -nostdsysteminc -std=c++20 -ferror-limit 0 -verify=expected,cxx20 C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\concepts.cpp
# executed command: 'c:\_work\llvm-project\llvm-project\build\bin\clang.exe' -cc1 -internal-isystem 'C:\_work\llvm-project\llvm-project\build\lib\clang\23\include' -nostdsysteminc -std=c++20 -ferror-limit 0 -verify=expected,cxx20 'C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\concepts.cpp'
# .---command stderr------------
# | error: diagnostics with 'error' severity expected but not seen: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\concepts.cpp Line 1723 'expected-error': expected ';' at end of requirement
# | error: diagnostics with 'error' severity seen but not expected: 
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\concepts.cpp Line 1722: expected ';' at end of requirement
# |   File C:\_work\llvm-project\llvm-project\clang\test\SemaTemplate\concepts.cpp Line 1722: expected ';' after return statement
# | 3 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/176896