[SourceKit] Add a new query to collect all group names from a module.

Input:
  Request Id: 'source.request.module.groups'
  Required field: 'key.modulename'

Output:
  Array of group names in that module, if any.

lib/Serialization/SerializedModuleLoader.cpp

@@ -494,6 +494,11 @@ SerializedASTFile::getGroupNameForDecl(const Decl *D) const {
   return File.getGroupNameForDecl(D);
 }
 
+void
+SerializedASTFile::collectAllGroups(std::vector<StringRef> &Names) const {
+  File.collectAllGroups(Names);
+};
+
 void
 SerializedASTFile::getTopLevelDecls(SmallVectorImpl<Decl*> &results) const {
   File.getTopLevelDecls(results);