[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/ModuleFile.cpp

@@ -156,7 +156,8 @@ ModuleFile::loadDependenciesForFileContext(const FileUnit *file,
     if (dependency.isHeader()) {
       // The path may be empty if the file being loaded is a partial AST,
       // and the current compiler invocation is a merge-modules step.
-      if (!dependency.Core.RawPath.empty()) {
+      if (!dependency.Core.RawPath.empty() &&
+          !M->getASTContext().SearchPathOpts.BridgingHeaderChaining) {
         bool hadError =
             clangImporter->importHeader(dependency.Core.RawPath,
                                         file->getParentModule(),