[clang] [clang][Serialization] Fix latent bug in adjustFilenameForRelocatableAST (PR #227910)

[via cfe-commits]

github-actions[bot] wrote:

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

* 56740 tests passed
* 1685 tests skipped
* 1 test failed

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

### Clang-Unit
<details>
<summary>Clang-Unit._/AllClangUnitTests_exe/ResolveImportedPathTest/RelativeToBase</summary>

```
Script:
--
C:\_work\llvm-project\llvm-project\build\tools\clang\unittests\.\AllClangUnitTests.exe --gtest_filter=ResolveImportedPathTest.RelativeToBase
--
C:\_work\llvm-project\llvm-project\clang\unittests\Serialization\ResolveImportedPathTest.cpp:26
Expected equality of these values:
  resolve("dir/file.h", "/base")
    Which is: "/base\\dir/file.h"
  Expected
    Which is: "/base\\dir\\file.h"


```
</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/227910