[clangd] Set CWD in semaCodeComplete

[jkorous]

A slightly tangential thing - we recently got an internal bugreport about clangd handling combination of working directory and relative paths for -fmodules-cache-path combination in compile_command.json incorrectly.
I tried some quick hacks but failed - it seems that ultimately the limiting factor is the code in clang dealing with modules not using VFS. It's on our list of things to fix although we don't have a timeframe for it.

[sammccall]

LGTM

clang-tools-extra/clangd/CodeComplete.cpp

bool semaCodeComplete(std::unique_ptr<CodeCompleteConsumer> Consumer,
   if (Input.Patch)
     Input.Patch->apply(*CI);
   // NOTE: we must call BeginSourceFile after prepareCompilerInstance. Otherwise
   // the remapped buffers do not get freed.
   llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem> VFS =
       Input.ParseInput.FSProvider->getFileSystem();
   if (Input.Preamble.StatCache)
     VFS = Input.Preamble.StatCache->getConsumingFS(std::move(VFS));
+  if (VFS->setCurrentWorkingDirectory(
+          Input.ParseInput.CompileCommand.Directory))
+    elog("Couldn't set working directory during code completion");
   auto Clang = prepareCompilerInstance(
       std::move(CI), !CompletingInPreamble ? &Input.Preamble.Preamble : nullptr,
       std::move(ContentsBuffer), std::move(VFS), IgnoreDiags);
   Clang->getPreprocessorOpts().SingleFileParseMode = CompletingInPreamble;
   Clang->setCodeCompletionConsumer(Consumer.release());
 
   SyntaxOnlyAction Action;
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0])) {