Print correctly dependency paths on Windows

Summary:
Before:
main.o: main.c ../include/lib\test.h

After:
main.o: main.c ../include/lib/test.h

Fixes PR38877

Reviewers: zturner

Subscribers: xbolva00, cfe-commits

Differential Revision: https://reviews.llvm.org/D51847

llvm-svn: 342139