[clangd][remote] Add Windows paths support

Without this patch 6 marshalling tests fail on Windows.
This patch contains the following changes:
- Allow paths with Windows slashes (convert to the POSIX style instead of assertion)
- 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.
- Port `RemoteMarshallingTest::IncludeHeaderURI` test to Windows.

Reviewed By: kbobyrev

Differential Revision: https://reviews.llvm.org/D89529