[Sema] Fix _Alignas/isCXX11Attribute() FIXME

When doing https://reviews.llvm.org/D148105 , I hadn't noticed that
there was also a FIXME about the misclassification of _Alignas in
ProcessDeclAttribute.

Differential Revision: https://reviews.llvm.org/D149148

clang/lib/Sema/SemaDeclAttr.cpp

@@ -8591,13 +8591,7 @@ ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D, const ParsedAttr &AL,
 
   // Ignore C++11 attributes on declarator chunks: they appertain to the type
   // instead.
-  // FIXME: We currently check the attribute syntax directly instead of using
-  // isCXX11Attribute(), which currently erroneously classifies the C11
-  // `_Alignas` attribute as a C++11 attribute. `_Alignas` can appear on the
-  // `DeclSpec`, so we need to let it through here to make sure it is processed
-  // appropriately. Once the behavior of isCXX11Attribute() is fixed, we can
-  // go back to using that here.
-  if (AL.getSyntax() == ParsedAttr::AS_CXX11 && !Options.IncludeCXX11Attributes)
+  if (AL.isCXX11Attribute() && !Options.IncludeCXX11Attributes)
     return;
 
   // Unknown attributes are automatically warned on. Target-specific attributes