Sema: Register clang's export_as in ModuleDecl::getExportAsModule

This service on ModuleDecl wasn't actually used before this PR. The main
client in ASTPrinter calls direclty the underlying logic in FileUnit.
Let's update it for our needs.

Author: xymus

lib/ClangImporter/ClangImporter.cpp

@@ -2781,6 +2781,9 @@ ClangModuleUnit *ClangImporter::Implementation::getWrapperForModule(
   wrapper->setIsSystemModule(underlying->IsSystem);
   wrapper->setIsNonSwiftModule();
   wrapper->setHasResolvedImports();
+  if (!underlying->ExportAsModule.empty())
+    wrapper->setExportAsName(
+        SwiftContext.getIdentifier(underlying->ExportAsModule));
 
   auto file = new (SwiftContext) ClangModuleUnit(*wrapper, *this,
                                                  underlying);