Rework getReexportedModules to optionally find all imported modules.

...instead of just those that are re-exported. This will be used for
autolinking (and probably few other places).

As part of this, we get two name changes:
  (1) Module::getReexportedModules -> getImportedModules
  (2) TranslationUnit::getImportedModules -> getImports

The latter doesn't just get modules-plus-access-paths; it also includes
whether or not the import is re-exported. Mainly, though, it just didn't
seem like a good idea to overload this name when the two functions aren't
really related.

No tests yet, will come with autolinking.

Swift SVN r7487

lib/Serialization/ModuleFile.cpp

@@ -494,10 +494,11 @@ OperatorDecl *ModuleFile::lookupOperator(Identifier name, DeclKind fixity) {
   return nullptr;
 }
 
-void ModuleFile::getReexportedModules(
-    SmallVectorImpl<Module::ImportedModule> &results) {
+void ModuleFile::getImportedModules(
+    SmallVectorImpl<Module::ImportedModule> &results,
+    bool includePrivate) {
   for (auto &dep : Dependencies) {
-    if (!dep.IsExported)
+    if (!includePrivate && !dep.IsExported)
       continue;
     assert(dep.isLoaded());
     results.push_back(dep.Import);