[clang][deps] Move enabling system header deps from `clang-scan-deps` to `DependencyScanning` library

[jansvoboda11]

Summary

This patch moves enabling system header deps from clang-scan-deps into the DependencyScanning library. This will make it easier to preserve semantics of the original TU command-line for modular dependencies (see D104036).

[arphaman]

Thanks, LGTM

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

bool runInvocation(std::shared_ptr<CompilerInvocation> Invocation,
     // and thus won't write out the extra '.d' files to disk.
     auto Opts = std::make_unique<DependencyOutputOptions>(
         std::move(Compiler.getInvocation().getDependencyOutputOpts()));
     // We need at least one -MT equivalent for the generator of make dependency
     // files to work.
     if (Opts->Targets.empty())
       Opts->Targets = {deduceDepTarget(Compiler.getFrontendOpts().OutputFile,
                                        Compiler.getFrontendOpts().Inputs)};
+    Opts->IncludeSystemHeaders = true;
 
     switch (Format) {
     case ScanningOutputFormat::Make:
       Compiler.addDependencyCollector(
           std::make_shared<DependencyConsumerForwarder>(std::move(Opts),
                                                         Consumer));
       break;
     case ScanningOutputFormat::Full:

clang/tools/clang-scan-deps/ClangScanDeps.cpp

AdjustingCompilations->appendArgumentsAdjuster(
         }
         tooling::CommandLineArguments AdjustedArgs(Args.begin(), FlagsEnd);
         // The clang-cl driver passes "-o -" to the frontend. Inject the real
         // file here to ensure "-MT" can be deduced if need be.
         if (ClangCLMode && !LastO.empty()) {
           AdjustedArgs.push_back("/clang:-o");
           AdjustedArgs.push_back("/clang:" + LastO);
         }
-        AdjustedArgs.push_back("-Xclang");
-        AdjustedArgs.push_back("-sys-header-deps");
 
         if (!HasResourceDir) {
           StringRef ResourceDir =
               ResourceDirCache.findResourceDir(Args, ClangCLMode);
           if (!ResourceDir.empty()) {
             AdjustedArgs.push_back("-resource-dir");
             AdjustedArgs.push_back(std::string(ResourceDir));
           }