Add description to assert

llvm-svn: 227291
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index f636142..07959a5 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -627,7 +627,7 @@
 
   DIType BaseType = DD->resolve(Ty.getTypeDerivedFrom());
 
-  assert(BaseType.isValid());
+  assert(BaseType.isValid() && "Unexpected invalid base type");
 
   // If this is a derived type, go ahead and get the base type, unless it's a
   // reference then it's just the size of the field. Pointer types have no need