[clang][deps] Disable global module index

[jansvoboda11]

Summary

While scanning dependencies of a TU that depends on a PCH, the scanner basically performs mixed implicit/explicit modular compilation. (Explicit modules come from the PCH.) This seems to trip up the global module index.

This patch disables global module index in the dependency scanner.

[jansvoboda11]

I'd like to add a test case, but actually creating a minimal one is proving to be difficult. This is most likely caused by the interaction between the global module index and explicit+implicit modules. Since we want to move off of the implicit build in dependency scanner, I don't think having a test case for this is crucial.

[Bigcheese]

lgtm. I agree that testing this isn't really necessary given the difficulty of even constructing one.

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

bool runInvocation(std::shared_ptr<CompilerInvocation> Invocation,
     sanitizeDiagOpts(ScanInstance.getDiagnosticOpts());
     ScanInstance.createDiagnostics(DiagConsumer, /*ShouldOwnClient=*/false);
     if (!ScanInstance.hasDiagnostics())
       return false;
 
     ScanInstance.getPreprocessorOpts().AllowPCHWithDifferentModulesCachePath =
         true;
 
+    ScanInstance.getFrontendOpts().GenerateGlobalModuleIndex = false;
+    ScanInstance.getFrontendOpts().UseGlobalModuleIndex = false;
+
     FileMgr->getFileSystemOpts().WorkingDir = std::string(WorkingDirectory);
     ScanInstance.setFileManager(FileMgr);
     ScanInstance.createSourceManager(*FileMgr);
 
     llvm::StringSet<> PrebuiltModulesInputFiles;
     // Store the list of prebuilt module files into header search options. This
     // will prevent the implicit build to create duplicate modules and will
     // force reuse of the existing prebuilt module files instead.