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

[Devon Loehr via llvm-commits]

DKLoehr wrote:

I've been experimenting with the simple option (canonicalize file names before matching, and require the patterns to use `/` as file separators on all systems), but it requires anyone who uses a case list with backslashes to rewrite it, which I suspect would be considered too onerous to merge.

If we don't impose that requirement, then we'd need to edit the globs as well, but I'm hesitant to mess with user-provided patterns because it seems easy to produce unintuitive behavior (and makes debugging harder on their end), e.g. they wonder why their `\` in a pattern isn't matching anything.

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