Always mark the initial REPL file as typechecked

We were previously doing this for the REPL, but
not for swift-ide-test. Move the assignment into
the frontend to make sure its always applied, and
inline `createREPLFile` while we're here.

Author: hamishknight

include/swift/Frontend/Frontend.h

@@ -653,8 +653,6 @@ class CompilerInstance {
   };
 
 private:
-  void createREPLFile(const ImplicitImports &implicitImports);
-
   void addMainFileToModule(const ImplicitImports &implicitImports);
 
   void performSemaUpTo(SourceFile::ASTStage_t LimitStage);

lib/Frontend/Frontend.cpp

@@ -727,7 +727,15 @@ void CompilerInstance::performSemaUpTo(SourceFile::ASTStage_t LimitStage) {
   const ImplicitImports implicitImports(*this);
 
   if (Invocation.getInputKind() == InputFileKind::SwiftREPL) {
-    createREPLFile(implicitImports);
+    // Create the initial empty REPL file. This only exists to feed in the
+    // implicit imports such as the standard library.
+    auto *replFile = createSourceFileForMainModule(
+        SourceFileKind::REPL, implicitImports.kind, /*BufferID*/ None);
+    addAdditionalInitialImportsTo(replFile, implicitImports);
+
+    // Given this file is empty, we can go ahead and just mark it as having been
+    // type checked.
+    replFile->ASTStage = SourceFile::TypeChecked;
     return;
   }
 
@@ -824,12 +832,6 @@ void CompilerInstance::getImplicitlyImportedModules(
   }
 }
 
-void CompilerInstance::createREPLFile(const ImplicitImports &implicitImports) {
-  auto *SingleInputFile = createSourceFileForMainModule(
-      Invocation.getSourceFileKind(), implicitImports.kind, None);
-  addAdditionalInitialImportsTo(SingleInputFile, implicitImports);
-}
-
 void CompilerInstance::addMainFileToModule(
     const ImplicitImports &implicitImports) {
   auto *MainFile = createSourceFileForMainModule(

lib/Immediate/REPL.cpp

@@ -997,10 +997,6 @@ class REPLEnvironment {
     IRGenOpts.DebugInfoLevel = IRGenDebugInfoLevel::None;
     IRGenOpts.DebugInfoFormat = IRGenDebugInfoFormat::None;
 
-    // The very first module is a dummy.
-    CI.getMainModule()->getMainSourceFile(SourceFileKind::REPL).ASTStage =
-        SourceFile::TypeChecked;
-
     if (!ParseStdlib) {
       // Force standard library to be loaded immediately.  This forces any
       // errors to appear upfront, and helps eliminate some nasty lag after the