Add SpellingNotCalculated to Attribute Enums to suppress UBSan warnings

UBSan downstreams noticed that the assignment of SpellingNotCalculated
to the spellings caused warnings.

llvm-svn: 372124
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
index e36f6932..e656fe6 100644
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2711,6 +2711,8 @@
         break;
       case MSInheritanceAttr::Keyword_unspecified_inheritance:
         break;
+      case MSInheritanceAttr::SpellingNotCalculated:
+        llvm_unreachable("Spelling not yet calculated");
       }
     }
   }