[clang-tools-extra] [clang-tidy] Fix broken HeaderFilterRegex when read from config file (PR #133582)

[Yat Ho via cfe-commits]

tearfur wrote:

Tested by applying this PR on top of 3e742b517a0606bfed329dfcb8c34c614ed73ea7. Works great AFAICT!

```console
# curl -sL 'https://github.com/llvm/llvm-project/pull/133582.diff' | git apply
```

https://github.com/llvm/llvm-project/pull/133582