[clang][deps] Disable global module index

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.

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D118890

clang/lib/Tooling/DependencyScanning/DependencyScanningWorker.cpp

@@ -177,6 +177,9 @@ class DependencyScanningAction : public tooling::ToolAction {
     ScanInstance.getPreprocessorOpts().AllowPCHWithDifferentModulesCachePath =
         true;
 
+    ScanInstance.getFrontendOpts().GenerateGlobalModuleIndex = false;
+    ScanInstance.getFrontendOpts().UseGlobalModuleIndex = false;
+
     FileMgr->getFileSystemOpts().WorkingDir = std::string(WorkingDirectory);
     ScanInstance.setFileManager(FileMgr);
     ScanInstance.createSourceManager(*FileMgr);