[Dependency Scanning] Share the Clang Dependency Scanning service among scanner invocations.

It was recently moved to the ModuleDependenciesCache. This is undesireable, instead this should live in the GlobalModuleDependenciesCache so that we benefit from the filesystem caching it performs across diferent scanning actions.

Author: artemcm

include/swift/AST/ModuleDependencies.h

@@ -502,6 +502,9 @@ class GlobalModuleDependenciesCache {
     ModuleDependenciesKindMap ModuleDependenciesMap;
   };
 
+  /// The 'persistent' Clang dependency scanner service
+  clang::tooling::dependencies::DependencyScanningService clangScanningService;
+
   /// All cached Swift source module dependencies, in the order in which they were encountered
   std::vector<ModuleDependencyID> AllSourceModules;
 
@@ -616,9 +619,6 @@ class ModuleDependenciesCache {
   StringRef mainScanModuleName;
   /// Set containing all of the Clang modules that have already been seen.
   llvm::StringSet<> alreadySeenClangModules;
-  /// The 'persistent' Clang dependency scanner service
-  /// TODO: Share this service among common scanner invocations
-  clang::tooling::dependencies::DependencyScanningService clangScanningService;
   /// The Clang dependency scanner tool
   clang::tooling::dependencies::DependencyScanningTool clangScanningTool;
 

lib/AST/ModuleDependencies.cpp

@@ -225,7 +225,15 @@ void ModuleDependencies::addBridgingModuleDependency(
   }
 }
 
-GlobalModuleDependenciesCache::GlobalModuleDependenciesCache() {}
+GlobalModuleDependenciesCache::GlobalModuleDependenciesCache()
+  : clangScanningService(
+                         clang::tooling::dependencies::ScanningMode::DependencyDirectivesScan,
+                         clang::tooling::dependencies::ScanningOutputFormat::Full,
+                         clang::CASOptions(),
+                         /* Cache */ nullptr,
+                         /* SharedFS */ nullptr,
+                         /* ReuseFileManager */ false,
+                         /* OptimizeArgs */ false) {}
 GlobalModuleDependenciesCache::TargetSpecificGlobalCacheState *
 GlobalModuleDependenciesCache::getCurrentCache() const {
   assert(CurrentTriple.hasValue() &&
@@ -536,15 +544,7 @@ ModuleDependenciesCache::ModuleDependenciesCache(
     StringRef mainScanModuleName)
     : globalCache(globalCache),
       mainScanModuleName(mainScanModuleName),
-      clangScanningService(
-          clang::tooling::dependencies::ScanningMode::DependencyDirectivesScan,
-          clang::tooling::dependencies::ScanningOutputFormat::Full,
-          clang::CASOptions(),
-          /* Cache */ nullptr,
-          /* SharedFS */ nullptr,
-          /* ReuseFileManager */ false,
-          /* OptimizeArgs */ false),
-      clangScanningTool(clangScanningService) {
+      clangScanningTool(globalCache.clangScanningService) {
   for (auto kind = ModuleDependenciesKind::FirstKind;
        kind != ModuleDependenciesKind::LastKind; ++kind) {
     ModuleDependenciesMap.insert(