[clangd] Fail inlayHints requests on content changes

[kadircet]

Summary

This should improve the overall UX by making the labels less jumpy.

[sammccall]

This seems right for consistency with SemanticHighlights.

However I don't think it will much improve the jumpiness you refer to - there's no time-based debounce here, so it only helps if you manage to bring the code from valid->invalid->valid without ever letting a request complete...

clang-tools-extra/clangd/ClangdServer.cpp

 void ClangdServer::inlayHints(PathRef File, llvm::Optional<Range> RestrictRange,
                               Callback<std::vector<InlayHint>> CB) {
   auto Action = [RestrictRange(std::move(RestrictRange)),
                  CB = std::move(CB)](Expected<InputsAndAST> InpAST) mutable {
     if (!InpAST)
       return CB(InpAST.takeError());
     CB(clangd::inlayHints(InpAST->AST, std::move(RestrictRange)));
   };
-  WorkScheduler->runWithAST("InlayHints", File, std::move(Action));
+  WorkScheduler->runWithAST("InlayHints", File, std::move(Action), Transient);
 }
 
 void ClangdServer::onFileEvent(const DidChangeWatchedFilesParams &Params) {
   // FIXME: Do nothing for now. This will be used for indexing and potentially
   // invalidating other caches.
 }
 
 void ClangdServer::workspaceSymbols(