[clangd] Compute the unused includes in the check mode.

[kadircet]

what's the motivating reason behind the change?
we'll soon change the implementation to use include-cleaner instead, and can run the tool directly to verify the findings (rather than clangd). hence i feel like this will just turn into not-so-useful extra output for clangd --check, but maybe there's some other use case that i am missing?

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

 #include "../clang-tidy/GlobList.h"
 #include "ClangdLSPServer.h"
 #include "CodeComplete.h"
 #include "CompileCommands.h"
 #include "Config.h"
 #include "Feature.h"
 #include "GlobalCompilationDatabase.h"
 #include "Hover.h"
+#include "IncludeCleaner.h"
 #include "InlayHints.h"
 #include "ParsedAST.h"
 #include "Preamble.h"
 #include "Protocol.h"
 #include "SemanticHighlighting.h"
 #include "SourceCode.h"
 #include "XRefs.h"
 #include "index/CanonicalIncludes.h"
for (const auto& Check : listTidyChecks(CheckTidyTime)) {
       log("  {0} = {1:P0}", Check, Fraction);
     }
     log("Finished individual clang-tidy checks");
 
     // Restore old options.
     Inputs.ClangTidyProvider = Opts.ClangTidyProvider;
   }
 
+  void computeUnusedIncludes() {
+    auto UnusedIncludes =  clang::clangd::computeUnusedIncludes(*AST);
+    log("Computing unused includes");
+    for (const auto *Unused : UnusedIncludes) {
+      log("  - {0} @Line:{1}", Unused->Written,
+          Unused->HashLine + 1 /*1-based*/);
+    }
+  }
+
   // Build Inlay Hints for the entire AST or the specified range
   void buildInlayHints(llvm::Optional<Range> LineRange) {
     log("Building inlay hints");
     auto Hints = inlayHints(*AST, LineRange);
 
     for (const auto &Hint : Hints) {
       vlog("  {0} {1} {2}", Hint.kind, Hint.position, Hint.label);
     }
WithContext Ctx(ContextProvider(
           ? File
           : /*Don't turn on local configs for an arbitrary temp path.*/ ""));
   Checker C(File, Opts);
   if (!C.buildCommand(TFS) || !C.buildInvocation(TFS, Contents) ||
       !C.buildAST())
     return false;
   C.buildInlayHints(LineRange);
   C.buildSemanticHighlighting(LineRange);
+  C.computeUnusedIncludes();
   if (CheckLocations)
     C.testLocationFeatures(LineRange);
 
   log("All checks completed, {0} errors", C.ErrCount);
   return C.ErrCount == 0;
 }
 
 } // namespace clangd
 } // namespace clang