Move vtordisp mode from Attr class to LangOptions.h, NFC

This removes one of the two uses of Attr.h in DeclCXX.h, reducing the
need to include Attr.h as widely. LangOptions is already very popular.
diff --git a/clang/lib/Parse/ParsePragma.cpp b/clang/lib/Parse/ParsePragma.cpp
index cdbf697..df411e1 100644
--- a/clang/lib/Parse/ParsePragma.cpp
+++ b/clang/lib/Parse/ParsePragma.cpp
@@ -734,7 +734,7 @@
   uintptr_t Value = reinterpret_cast<uintptr_t>(Tok.getAnnotationValue());
   Sema::PragmaMsStackAction Action =
       static_cast<Sema::PragmaMsStackAction>((Value >> 16) & 0xFFFF);
-  MSVtorDispAttr::Mode Mode = MSVtorDispAttr::Mode(Value & 0xFFFF);
+  MSVtorDispMode Mode = MSVtorDispMode(Value & 0xFFFF);
   SourceLocation PragmaLoc = ConsumeAnnotationToken();
   Actions.ActOnPragmaMSVtorDisp(Action, PragmaLoc, Mode);
 }