[BrdigingHeader] Auto bridging header chaining

Add ability to automatically chaining the bridging headers discovered from all
dependencies module when doing swift caching build. This will eliminate all
implicit bridging header imports from the build and make the bridging header
importing behavior much more reliable, while keep the compatibility at maximum.

For example, if the current module A depends on module B and C, and both B and
C are binary modules that uses bridging header, when building module A,
dependency scanner will construct a new header that chains three bridging
headers together with the option to build a PCH from it. This will make all
importing errors more obvious while improving the performance.

include/swift/DependencyScan/ModuleDependencyScanner.h

@@ -16,6 +16,7 @@
 #include "swift/AST/ModuleDependencies.h"
 #include "swift/Frontend/ModuleInterfaceLoader.h"
 #include "swift/Serialization/SerializedModuleLoader.h"
+#include "llvm/CAS/CASReference.h"
 #include "llvm/Support/ThreadPool.h"
 
 namespace swift {
@@ -48,6 +49,11 @@ private:
                                          llvm::PrefixMapper *prefixMapper,
                                          bool isTestableImport = false);
 
+  /// Store cache entry for include tree.
+  llvm::Error
+  createCacheKeyForEmbeddedHeader(std::string embeddedHeaderIncludeTree,
+                                  std::string chainedHeaderIncludeTree);
+
   // Worker-specific instance of CompilerInvocation
   std::unique_ptr<CompilerInvocation> workerCompilerInvocation;
   // Worker-specific instance of ASTContext
@@ -59,6 +65,9 @@ private:
   // Swift and Clang module loaders acting as scanners.
   std::unique_ptr<ModuleInterfaceLoader> swiftScannerModuleLoader;
   std::unique_ptr<ClangImporter> clangScannerModuleLoader;
+  // CAS instance.
+  std::shared_ptr<llvm::cas::ObjectStore> CAS;
+  std::shared_ptr<llvm::cas::ActionCache> ActionCache;
   // Restrict access to the parent scanner class.
   friend class ModuleDependencyScanner;
 };
@@ -116,10 +125,9 @@ private:
   resolveAllClangModuleDependencies(ArrayRef<ModuleDependencyID> swiftModules,
                                     ModuleDependenciesCache &cache,
                                     ModuleDependencyIDSetVector &discoveredClangModules);
-  void
-  resolveHeaderDependencies(ArrayRef<ModuleDependencyID> swiftModules,
-                            ModuleDependenciesCache &cache,
-                            ModuleDependencyIDSetVector &discoveredHeaderDependencyClangModules);
+  void resolveHeaderDependencies(
+      ArrayRef<ModuleDependencyID> swiftModules, ModuleDependenciesCache &cache,
+      ModuleDependencyIDSetVector &discoveredHeaderDependencyClangModules);
   void
   resolveSwiftOverlayDependencies(ArrayRef<ModuleDependencyID> swiftModules,
                                   ModuleDependenciesCache &cache,
@@ -151,6 +159,12 @@ private:
       ModuleDependenciesCache &cache,
       llvm::function_ref<void(ModuleDependencyID)> action);
 
+  /// Performance BridgingHeader Chaining.
+  llvm::Error
+  performBridgingHeaderChaining(const ModuleDependencyID &rootModuleID,
+                                ModuleDependenciesCache &cache,
+                                ModuleDependencyIDSetVector &allModules);
+
   /// Perform an operation utilizing one of the Scanning workers
   /// available to this scanner.
   template <typename Function, typename... Args>