[clang] [llvm] Make sanitizer special case list slash-agnostic (PR #149886)

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

> Keeping backward compatibility in ignore lists looks like unnecessary maintenance burden. Migration is easy, but if we not doing this, we stuck with that for years.

BTW. We want to do another breaking change in this version.

```
src:./path/
src:path/
```

Warning file does this righght and removes ./,
I'd like to expand this on all lists.

So we can bundle those changes them together.

https://github.com/llvm/llvm-project/pull/149886