Swap InputFileKind for ParseInputMode

Tying InputFile to this option meant that every input that was not one of the explictly-blessed kinds was modeled as a Swift file.

With the new InputFile that infers file kinds, we no longer need CompilerInvocation::setInputKind

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1142,7 +1142,6 @@ void InterfaceSubContextDelegateImpl::inheritOptionsForBuildingInterface(
     genericSubInvocation.setSDKPath(SearchPathOpts.SDKPath);
   }
 
-  genericSubInvocation.setInputKind(InputFileKind::SwiftModuleInterface);
   if (!SearchPathOpts.RuntimeResourcePath.empty()) {
     genericSubInvocation.setRuntimeResourcePath(SearchPathOpts.RuntimeResourcePath);
   }