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

[Hans Wennborg via llvm-commits]

zmodem wrote:

3) Sounds best to me as well. (And I think the canonization should run regardless of host OS.)

Backwards compatibility is a concern though. Especially for projects that wish to build with either Clang 21 or 22, this might not be so nice.

https://clang.llvm.org/docs/SanitizerSpecialCaseList.html#format mentions a previous format transition. Maybe we should do some variant of `#!special-case-list-v1` for this change too? (`#!canonical-paths`?)

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