Without this patch 7 marshalling tests fails on Windows.
This patch contains the following changes:
- Allow paths with Windows slashes.
- Add support for URI with Windows path.
- Change the value of the second parameter of several `llvm::sys::path::convert_to_slash()` calls:
We should use `windows` instead of `posix` to ensure UNIX slashes in the path.
- Remove part of `RemoteMarshallingTest::IncludeHeaderURI` test which could not be ported on Windows.