[clangd] Don't ignore headers with IWYU export pragmas for unused-include analysis.

[hokein]

Summary

This check was introduced due to the the lack support of the old implementation.
The new include-cleaner-library based implementation doesn't have this
limitation.

[hokein]

It appears that there is an issue with this patch, so please hold on for review.

clang-tools-extra/clangd/IncludeCleaner.cpp

static bool mayConsiderUnused(const Inclusion &Inc, ParsedAST &AST,
   // Until we have good support for umbrella headers, don't warn about them.
   if (Inc.Written.front() == '<') {
     if (AnalyzeStdlib && tooling::stdlib::Header::named(Inc.Written))
       return true;
     return false;
   }
   assert(Inc.HeaderID);
   auto HID = static_cast<IncludeStructure::HeaderID>(*Inc.HeaderID);
-  // FIXME: Ignore the headers with IWYU export pragmas for now, remove this
-  // check when we have more precise tracking of exported headers.
-  if (AST.getIncludeStructure().hasIWYUExport(HID))
-    return false;
   auto FE = AST.getSourceManager().getFileManager().getFileRef(
       AST.getIncludeStructure().getRealPath(HID));
   assert(FE);
   // Headers without include guards have side effects and are not
   // self-contained, skip them.
   if (!AST.getPreprocessor().getHeaderSearchInfo().isFileMultipleIncludeGuarded(
           &FE->getFileEntry())) {
     dlog("{0} doesn't have header guard and will not be considered unused",

clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp

TU.AdditionalFiles["foo.h"] = R"cpp(
     #endif
   )cpp";
   TU.AdditionalFiles["bar.h"] = guard(R"cpp(
     void bar() {}
   )cpp");
   ParsedAST AST = TU.build();
 
   EXPECT_THAT(AST.getDiagnostics(), llvm::ValueIs(IsEmpty()));
-  // FIXME: This is not correct: foo.h is unused but is not diagnosed as such
-  // because we ignore headers with IWYU export pragmas for now.
   IncludeCleanerFindings Findings = computeIncludeCleanerFindings(AST);
-  EXPECT_THAT(Findings.UnusedIncludes, IsEmpty());
+  EXPECT_THAT(Findings.UnusedIncludes,
+              ElementsAre(Pointee(writtenInclusion("\"foo.h\""))));
 }
 
 TEST(IncludeCleaner, NoDiagsForObjC) {
   TestTU TU;
   TU.Code = R"cpp(
     #include "foo.h"
 
     void bar() {}