[ParseableInterface] Turn on -enable-parseable-module-interface always (#23331)

...and remove the option. This is ~technically~ CLI-breaking because
Swift 5 shipped this as a hidden driver option, but it wouldn't have
/done/ anything in Swift 5, so I think it's okay to remove.

Note that if a parseable interface (.swiftinterface) and a binary
interface (.swiftmodule) are both present, the binary one will still
be preferred. This just /allows/ parseable interfaces to be used.

rdar://problem/36885834

lib/Frontend/ParseableInterfaceModuleLoader.cpp

@@ -258,7 +258,6 @@ class swift::ParseableInterfaceBuilder {
   void configureSubInvocationInputsAndOutputs(StringRef OutPath) {
     auto &SubFEOpts = subInvocation.getFrontendOptions();
     SubFEOpts.RequestedAction = FrontendOptions::ActionType::EmitModuleOnly;
-    SubFEOpts.EnableParseableModuleInterface = true;
     SubFEOpts.InputsAndOutputs.addPrimaryInputFile(interfacePath);
     SupplementaryOutputPaths SOPs;
     SOPs.ModuleOutputPath = OutPath.str();
@@ -713,9 +712,6 @@ class ParseableInterfaceModuleLoaderImpl {
     if (validationInfo.status != serialization::Status::Valid)
       return false;
 
-    assert(validationInfo.name == moduleName &&
-           "we built a module at this path with a different name?");
-
     return dependenciesAreUpToDate(allDeps);
   }