[clangd][remote] Make sure relative paths are absolute with respect t…

…o posix style

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.

Differential Revision: https://reviews.llvm.org/D88507

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

@@ -299,7 +299,7 @@ Marshaller::relativePathToURI(llvm::StringRef RelativePath) {
   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);