Add missing const qualifier to member function Decl::isLocalExternDecl()

[firolino]

Summary

This patch simply adds a const qualifier to the member function Decl::isLocalExternDecl().

[ahatanak]

This looks OK to commit.

include/clang/AST/DeclBase.h

void setLocalExternDecl() {
     IdentifierNamespace |= IDNS_LocalExtern;
     if (Prev && Prev->getIdentifierNamespace() & IDNS_Ordinary)
       IdentifierNamespace |= IDNS_Ordinary;
   }
 
   /// \brief Determine whether this is a block-scope declaration with linkage.
   /// This will either be a local variable declaration declared 'extern', or a
   /// local function declaration.
-  bool isLocalExternDecl() {
+  bool isLocalExternDecl() const {
     return IdentifierNamespace & IDNS_LocalExtern;
   }
 
   /// \brief Changes the namespace of this declaration to reflect that it's
   /// the object of a friend declaration.
   ///
   /// These declarations appear in the lexical context of the friending
   /// class, but in the semantic context of the actual entity.  This property