[clang][deps] Add module files for input dependencies earlier

[benlangmuir]

Summary

I originally thought we needed to add module file inputs for modular deps at the same time as outputs because they depend on the lookupModuleOutput callback, but this is not the case: they only depend on the callback results for other modules, which have already been computed by this point. So move them earlier so that they're set in the CompilerInvocation at the same time as other inputs. This makes the code easier to understand.

This change is effectively NFC, though it technically changes the module exact value of the context hash.

[jansvoboda11]

LGTM.

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

static std::vector<std::string> splitString(std::string S, char Separator) {
   Result.reserve(Segments.size());
   for (StringRef Segment : Segments)
     Result.push_back(Segment.str());
   return Result;
 }
 
 void ModuleDepCollector::addOutputPaths(CompilerInvocation &CI,
                                         ModuleDeps &Deps) {
-  // These are technically *inputs* to the compilation, but we populate them
-  // here in order to make \c getModuleContextHash() independent of
-  // \c lookupModuleOutput().
-  addModuleFiles(CI, Deps.ClangModuleDeps);
-
   CI.getFrontendOpts().OutputFile =
       Consumer.lookupModuleOutput(Deps.ID, ModuleOutputKind::ModuleFile);
   if (!CI.getDiagnosticOpts().DiagnosticSerializationFile.empty())
     CI.getDiagnosticOpts().DiagnosticSerializationFile =
         Consumer.lookupModuleOutput(
             Deps.ID, ModuleOutputKind::DiagnosticSerializationFile);
   if (!CI.getDependencyOutputOpts().OutputFile.empty()) {
     CI.getDependencyOutputOpts().OutputFile =
for (StringRef ModuleMapFile : Deps.ModuleMapFileDeps) {
 
     CI.getFrontendOpts().ModuleMapFiles.emplace_back(ModuleMapFile);
   }
 
   // Report the prebuilt modules this module uses.
   for (const auto &PrebuiltModule : Deps.PrebuiltModuleDeps)
     CI.getFrontendOpts().ModuleFiles.push_back(PrebuiltModule.PCMFile);
 
+  // Add module file inputs from dependencies.
+  addModuleFiles(CI, Deps.ClangModuleDeps);
+
   // Remove any macro definitions that are explicitly ignored.
   if (!CI.getHeaderSearchOpts().ModulesIgnoreMacros.empty()) {
     llvm::erase_if(
         CI.getPreprocessorOpts().Macros,
         [&CI](const std::pair<std::string, bool> &Def) {
           StringRef MacroDef = Def.first;
           return CI.getHeaderSearchOpts().ModulesIgnoreMacros.contains(
               llvm::CachedHashString(MacroDef.split('=').first));