Add option to disable module loading.

This patch was created by Lawrence Crowl and reviewed in:
http://llvm-reviews.chandlerc.com/D963

llvm-svn: 187738
diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index 66f23f1..035f751 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -734,7 +734,7 @@
   }
 
   // If we have a non-empty module path, load the named module.
-  if (!ModuleImportPath.empty()) {
+  if (!ModuleImportPath.empty() && getLangOpts().Modules) {
     Module *Imported = TheModuleLoader.loadModule(ModuleImportLoc,
                                                   ModuleImportPath,
                                                   Module::MacrosVisible,