[clangd] Add std::move() to a return statement to please some compilers.

[mboehme]

Summary

This has been causing build errors in Swift CI.

clang-tools-extra/clangd/index/FileIndex.cpp

FileShardedIndex::getShard(llvm::StringRef Uri) const {
   }
   IF.Refs = std::move(RefB).build();
 
   RelationSlab::Builder RelB;
   for (const auto *Rel : It->getValue().Relations) {
     RelB.insert(*Rel);
   }
   IF.Relations = std::move(RelB).build();
-  return IF;
+  // Explicit move here is needed by some compilers.
+  return std::move(IF);
 }
 
 SlabTuple indexMainDecls(ParsedAST &AST) {
   return indexSymbols(AST.getASTContext(), AST.getPreprocessorPtr(),
                       AST.getLocalTopLevelDecls(), &AST.getMacros(),
                       AST.getCanonicalIncludes(),
                       /*IsIndexMainAST=*/true, AST.version());
 }