[libprofile] Handle '\\' in __llvm_profile_recursive_mkdir

This is implicitly needed at least by gcc-flag-compatibility.test

The thing that needs it is the `\` preceding the "default.profraw"
appended internally by clang when doing `-fprofile-use=`.

Clang uses `\` because is uses sys::path::append which will use `\` on a
Windows host. This is wrong, but I don't think there's an easy way to
solve it (maybe just always using `/` since places that accept `\` also
tend to accept `/`, but not the other way around).

llvm-svn: 264665