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/Sema/Sema.cpp b/clang/lib/Sema/Sema.cpp
index c3c6cad..035cb2e 100644
--- a/clang/lib/Sema/Sema.cpp
+++ b/clang/lib/Sema/Sema.cpp
@@ -137,7 +137,7 @@
       OriginalLexicalContext(nullptr), MSStructPragmaOn(false),
       MSPointerToMemberRepresentationMethod(
           LangOpts.getMSPointerToMemberRepresentationMethod()),
-      VtorDispStack(MSVtorDispAttr::Mode(LangOpts.VtorDispMode)), PackStack(0),
+      VtorDispStack(LangOpts.getVtorDispMode()), PackStack(0),
       DataSegStack(nullptr), BSSSegStack(nullptr), ConstSegStack(nullptr),
       CodeSegStack(nullptr), CurInitSeg(nullptr), VisContext(nullptr),
       PragmaAttributeCurrentTargetDecl(nullptr),