clang/lib/Driver/Multilib.cpp: Rewrite normalizePathSegment() to be tolerant dosish pathsep.

FIXME: It could be more simple...
clang/test/Modules/fmodules-validate-once-per-build-session.c: Tweak for gnuwin32 with %/t (instead of %t).

xargs didn't handle dosish paths, while find foo\bar\ emits dosish paths.