[clang] [clang][Dependency Scanning] Refactor Scanning Compiler Instance Initialization (PR #161300)

[Jan Svoboda via cfe-commits]

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@@ -434,14 +535,39 @@ bool DependencyScanningAction::runInvocation(
 
   ScanInstance.createSourceManager(*FileMgr);
 
+  // Consider different header search and diagnostic options to create
+  // different modules. This avoids the unsound aliasing of module PCMs.
+  //
+  // TODO: Implement diagnostic bucketing to reduce the impact of strict
+  // context hashing.
+  ScanInstance.getHeaderSearchOpts().ModulesStrictContextHash = true;
+  ScanInstance.getHeaderSearchOpts().ModulesSerializeOnlyPreprocessor = true;
+  ScanInstance.getHeaderSearchOpts().ModulesSkipDiagnosticOptions = true;
+  ScanInstance.getHeaderSearchOpts().ModulesSkipHeaderSearchPaths = true;
+  ScanInstance.getHeaderSearchOpts().ModulesSkipPragmaDiagnosticMappings = true;
+  ScanInstance.getHeaderSearchOpts().ModulesForceValidateUserHeaders = false;
+
+  // Avoid some checks and module map parsing when loading PCM files.
+  ScanInstance.getPreprocessorOpts().ModulesCheckRelocated = false;
+
+  return true;
+}
+
+llvm::SmallVector<StringRef> clang::tooling::dependencies::computeStableDirs(
+    CompilerInstance &ScanInstance) {
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jansvoboda11 wrote:

```suggestion
    const CompilerInstance &ScanInstance) {
```

https://github.com/llvm/llvm-project/pull/161300