Fix missing return on error path in loadExecutablePlugin().

Author: DougGregor

lib/AST/ASTContext.cpp

@@ -6407,6 +6407,7 @@ LoadedExecutablePlugin *ASTContext::loadExecutablePlugin(StringRef path) {
   if (!plugin) {
     Diags.diagnose(SourceLoc(), diag::compiler_plugin_not_loaded, path,
                    llvm::toString(plugin.takeError()));
+    return nullptr;
   }
 
   return plugin.get();