Make decl shadowing ask the right question

If it's going to validate the declaration, it wants to know if there's an interface type.  Computing the generic signature can be done on demand.

Author: CodaFi

lib/AST/NameLookup.cpp

@@ -470,7 +470,7 @@ static void recordShadowedDecls(ArrayRef<ValueDecl *> decls,
       // If the decl is currently being validated, this is likely a recursive
       // reference and we'll want to skip ahead so as to avoid having its type
       // attempt to desugar itself.
-      if (!decl->hasValidSignature())
+      if (!decl->hasInterfaceType())
         continue;
 
       // FIXME: the canonical type makes a poor signature, because we don't