[Sema] Don't skip import resolution for empty file

The associated comment appears to outdated. Make
sure we continue to record implicit imports for
empty files.

Author: hamishknight

lib/Sema/ImportResolution.cpp

@@ -276,17 +276,12 @@ class ImportResolver final : public DeclVisitor<ImportResolver> {
 ///
 /// Import resolution operates on a parsed but otherwise unvalidated AST.
 void swift::performImportResolution(SourceFile &SF) {
-  FrontendStatsTracer tracer(SF.getASTContext().Stats,
-                             "Import resolution");
-
-  // Make sure we skip adding the standard library imports if the
-  // source file is empty.
-  if (SF.ASTStage == SourceFile::ImportsResolved ||
-      SF.getTopLevelDecls().empty()) {
-    SF.ASTStage = SourceFile::ImportsResolved;
+  // If we've already performed import resolution, bail.
+  if (SF.ASTStage == SourceFile::ImportsResolved)
     return;
-  }
 
+  FrontendStatsTracer tracer(SF.getASTContext().Stats,
+                             "Import resolution");
   ImportResolver resolver(SF);
 
   // Resolve each import declaration.