Revert "Introduce a TypeSystem interface to support adding non-clang languages."

This seems to break expression evaluation on the linux build.

llvm-svn: 239366
diff --git a/lldb/source/DataFormatters/NSIndexPath.cpp b/lldb/source/DataFormatters/NSIndexPath.cpp
index 363069f..6932211 100644
--- a/lldb/source/DataFormatters/NSIndexPath.cpp
+++ b/lldb/source/DataFormatters/NSIndexPath.cpp
@@ -49,10 +49,7 @@
     {
         m_impl.m_mode = Mode::Invalid;
         
-        TypeSystem* type_system = m_backend.GetClangType().GetTypeSystem();
-        if (!type_system)
-            return false;
-        m_ast_ctx = type_system->AsClangASTContext();
+        m_ast_ctx = ClangASTContext::GetASTContext(m_backend.GetClangType().GetASTContext());
         if (!m_ast_ctx)
             return false;