[clang] [clang][test] Fix test failures when LLVM_WINDOWS_PREFER_FORWARD_SLAS… (PR #193145)

[via cfe-commits]

github-actions[bot] wrote:

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

* 87565 tests passed
* 1476 tests skipped
* 2 tests failed

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

### Clang-Unit
<details>
<summary>Clang-Unit._/AllClangUnitTests/ToolChainTest/ConfigInexistentInclude</summary>

```
Script:
--
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/unittests/./AllClangUnitTests --gtest_filter=ToolChainTest.ConfigInexistentInclude
--
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/unittests/Driver/ToolChainTest.cpp:846
Expected equality of these values:
  "cannot read configuration file 'C:/home/user/test.cfg" "': cannot not open file 'C:/home/user/file.rsp" "': no such file or directory"
    Which is: "cannot read configuration file 'C:/home/user/test.cfg': cannot not open file 'C:/home/user/file.rsp': no such file or directory"
  Error.c_str()
    Which is: "cannot read configuration file '/home/user/test.cfg': cannot not open file '/home/user/file.rsp': No such file or directory"
Ignoring case


```
</details>
<details>
<summary>Clang-Unit._/AllClangUnitTests/ToolChainTest/ConfigRecursiveInclude</summary>

```
Script:
--
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/tools/clang/unittests/./AllClangUnitTests --gtest_filter=ToolChainTest.ConfigRecursiveInclude
--
/home/gha/actions-runner/_work/llvm-project/llvm-project/clang/unittests/Driver/ToolChainTest.cpp:883
Expected equality of these values:
  "cannot read configuration file 'C:/home/user/test.cfg" "': recursive expansion of: 'C:/home/user/file1.cfg'"
    Which is: "cannot read configuration file 'C:/home/user/test.cfg': recursive expansion of: 'C:/home/user/file1.cfg'"
  Error.c_str()
    Which is: "cannot read configuration file '/home/user/test.cfg': recursive expansion of: '/home/user/file1.cfg'"


```
</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/193145