ModuleInterface: refactor ModuleInterfaceChecker out of ModuleInterfaceLoader

This refactoring allows us to drop ModuleInterfaceLoader when explicit modules
are enabled. Before this change, the dependencies scanner needs the loader to be
present to access functionalities like collecting prebuilt module candidates.

lib/Frontend/ModuleInterfaceBuilder.cpp

@@ -170,10 +170,9 @@ bool ModuleInterfaceBuilder::buildSwiftModuleInternal(
     auto &SubInstance = *info.Instance;
     auto subInvocation = SubInstance.getInvocation();
     // Try building forwarding module first. If succeed, return.
-    if (static_cast<ModuleInterfaceLoader*>(SubInstance.getASTContext()
-        .getModuleInterfaceLoader())->tryEmitForwardingModule(moduleName,
-                                                              interfacePath,
-                                                  CompiledCandidates, OutPath)) {
+    if (SubInstance.getASTContext().getModuleInterfaceChecker()
+          ->tryEmitForwardingModule(moduleName, interfacePath,
+                                    CompiledCandidates, OutPath)) {
       return std::error_code();
     }
     FrontendOptions &FEOpts = subInvocation.getFrontendOptions();