[clang] [clang] Prevent __attribute__((diagnose_if(..., "error"))) from altering program behavior under SFINAE (PR #229858)

[via cfe-commits]

github-actions[bot] wrote:

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

* 61877 tests passed
* 2024 tests skipped
* 1 test failed

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

### Clang
<details>
<summary>Clang.SemaCXX/attribute-pack-expansion.cpp</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/24/include -nostdsysteminc -std=c++20 -fsyntax-only -verify /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/SemaCXX/attribute-pack-expansion.cpp
# 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/24/include -nostdsysteminc -std=c++20 -fsyntax-only -verify /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/SemaCXX/attribute-pack-expansion.cpp
# .---command stderr------------
# | error: 'expected-error' diagnostics seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/SemaCXX/attribute-pack-expansion.cpp Line 13: foobar
# | error: 'expected-note' diagnostics seen but not expected: 
# |   File /home/gha/actions-runner/_work/llvm-project/llvm-project/clang/test/SemaCXX/attribute-pack-expansion.cpp Line 13: from 'diagnose_if' attribute on 'operator()':
# | 2 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/229858