[clangd] Set FileSystem for tweaks in Check tool.

[usaxena95]

Summary

Tweaks like DefineOutline depend on FS to be set at apply() time.
After https://reviews.llvm.org/D93978, tweaks run from Check tool lost
access to FS. This makes the available to apply() once again.

[kadircet]

thanks, lgtm!

clang-tools-extra/clangd/tool/Check.cpp

215–221

IIUC, the ordering here is to ensure FS is not available during prepare but exists in apply. That sounds sane, as we would like prepare to be fast, and IO is likely slow.
But this is very subtle, so deserves a comment.

[usaxena95]

added comment.

[usaxena95]

Did you need any other change ? Didn't realise the revision was not "accepted".

[kadircet]

oops, sorry. i thought i've accepted the revision already :D

clang-tools-extra/clangd/tool/Check.cpp

for (const auto &Tok : SpelledTokens) {
 
       // FIXME: dumping the tokens may leak sensitive code into bug reports.
       // Add an option to turn this off, once we decide how options work.
       vlog("  {0} {1}", Pos, Tok.text(AST->getSourceManager()));
       auto Tree = SelectionTree::createRight(AST->getASTContext(),
                                              AST->getTokens(), Start, End);
       Tweak::Selection Selection(&Index, *AST, Start, End, std::move(Tree),
                                  nullptr);
-      for (const auto &T :
-           prepareTweaks(Selection, Opts.TweakFilter, Opts.FeatureModules)) {
+      // FS is only populated when applying a tweak, not during prepare as
+      // prepare should not do any I/O to be fast.
+      auto Tweaks =
+          prepareTweaks(Selection, Opts.TweakFilter, Opts.FeatureModules);
+      Selection.FS =
+          &AST->getSourceManager().getFileManager().getVirtualFileSystem();
+      for (const auto &T : Tweaks) {
         auto Result = T->apply(Selection);
         if (!Result) {
           elog("    tweak: {0} ==> FAIL: {1}", T->id(), Result.takeError());
           ++ErrCount;
         } else {
           vlog("    tweak: {0}", T->id());
         }
       }