[clangd][remote] Make sure relative paths are absolute with respect to posix style

[kadircet]

Summary

Relative paths received from the server are always in posix style. So
we need to ensure they are relative using that style, and not the native one.

[kbobyrev]

Ahh, good catch! Thank you!

clang-tools-extra/clangd/index/remote/marshalling/Marshalling.cpp

 }
 
 llvm::Expected<std::string>
 Marshaller::relativePathToURI(llvm::StringRef RelativePath) {
   assert(LocalIndexRoot);
   assert(RelativePath == llvm::sys::path::convert_to_slash(RelativePath));
   if (RelativePath.empty())
     return error("Empty relative path.");
-  if (llvm::sys::path::is_absolute(RelativePath))
+  if (llvm::sys::path::is_absolute(RelativePath, llvm::sys::path::Style::posix))
     return error("RelativePath '{0}' is absolute.", RelativePath);
   llvm::SmallString<256> FullPath = llvm::StringRef(*LocalIndexRoot);
   llvm::sys::path::append(FullPath, RelativePath);
   auto Result = URI::createFile(FullPath);
   return Result.toString();
 }
 
 llvm::Expected<std::string> Marshaller::uriToRelativePath(llvm::StringRef URI) {