[AST] Delete the unused FileUnitKind::Derived.

The last use of this went away a loooooong time ago. NFC.

Author: DougGregor

include/swift/AST/Module.h

@@ -94,8 +94,6 @@ enum class FileUnitKind {
   SerializedAST,
   /// An imported Clang module.
   ClangModule,
-  /// A derived declaration.
-  Derived,
 };
 
 enum class SourceFileKind {

lib/AST/DeclContext.cpp

@@ -558,9 +558,6 @@ unsigned DeclContext::printContext(raw_ostream &OS, unsigned indent) const {
     case FileUnitKind::Builtin:
       OS << " Builtin";
       break;
-    case FileUnitKind::Derived:
-      OS << " derived";
-      break;
     case FileUnitKind::Source:
       OS << " file=\"" << cast<SourceFile>(this)->getFilename() << "\"";
       break;

lib/AST/Module.cpp

@@ -835,8 +835,7 @@ lookupOperatorDeclForName(const FileUnit &File, SourceLoc Loc, Identifier Name,
 {
   switch (File.getKind()) {
   case FileUnitKind::Builtin:
-  case FileUnitKind::Derived:
-    // The Builtin module declares no operators, nor do derived units.
+    // The Builtin module declares no operators.
     return nullptr;
   case FileUnitKind::Source:
     break;

lib/IDE/CodeCompletion.cpp

@@ -3227,7 +3227,6 @@ class CompletionLookup final : public swift::VisibleDeclConsumer {
       for (auto fileUnit : import.second->getFiles()) {
         switch (fileUnit->getKind()) {
         case FileUnitKind::Builtin:
-        case FileUnitKind::Derived:
         case FileUnitKind::ClangModule:
           continue;
         case FileUnitKind::Source:

lib/Index/Index.cpp

@@ -573,7 +573,6 @@ bool IndexSwiftASTWalker::visitImports(
       switch (File->getKind()) {
       case FileUnitKind::Source:
       case FileUnitKind::Builtin:
-      case FileUnitKind::Derived:
         break;
       case FileUnitKind::SerializedAST:
         assert(!IsClangModuleOpt.hasValue() &&
@@ -1282,9 +1281,6 @@ void IndexSwiftASTWalker::getRecursiveModuleImports(
           case FileUnitKind::Builtin:
             Info += "builtin";
             break;
-          case FileUnitKind::Derived:
-            Info += "derived";
-            break;
           case FileUnitKind::Source:
             Info += "source, file=\"";
             Info += cast<SourceFile>(FU)->getFilename();

lib/Index/IndexRecord.cpp

@@ -410,7 +410,6 @@ static void addModuleDependencies(ArrayRef<ModuleDecl::ImportedModule> imports,
     for (auto *FU : mod->getFiles()) {
       switch (FU->getKind()) {
       case FileUnitKind::Source:
-      case FileUnitKind::Derived:
       case FileUnitKind::Builtin:
         break;
       case FileUnitKind::SerializedAST: