[DebugInfo] Fix file path separator when targeting windows.

This fixes two problems:

1. When crossing compiling for windows on linux, source file path in debug info is concatenated with directory by host native separator ('/'). For windows local build, they are concatenated by '\'. This causes non-determinism bug.

  The solution here is to let `LangOptions.UseTargetPathSeparator` to control if we should use host native separator or not.

2. Objectfile path in CodeView also uses host native separator when generated.

 It's fixed by changing the path separator in `/Fo` to '\' if the path is not an absolute path when adding the `-object-file-name=` flag.

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D147256