[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.

lib/Serialization/ModuleFileSharedCore.h

@@ -601,7 +601,7 @@ public:
 
   /// Outputs information useful for diagnostics to \p out
   void outputDiagnosticInfo(llvm::raw_ostream &os) const;
-  
+
   // Out of line to avoid instantiation OnDiskChainedHashTable here.
   ~ModuleFileSharedCore();
 
@@ -653,6 +653,12 @@ public:
     return MacroModuleNames;
   }
 
+  /// Get embedded bridging header.
+  std::string getEmbeddedHeader() const {
+    // Don't include the '\0' in the end.
+    return importedHeaderInfo.contents.drop_back().str();
+  }
+
   /// If the module-defining `.swiftinterface` file is an SDK-relative path,
   /// resolve it to be absolute to the specified SDK.
   std::string resolveModuleDefiningFilePath(const StringRef SDKPath) const;