When parsing a C++17 binding declaration, we first create the
BindingDecls in Sema::ActOnDecompositionDeclarator, and then build the
DecompositionDecl in Sema::ActOnVariableDeclarator, so the contained
BindingDecls are never null. But when deserializing, we read the
DecompositionDecl with all properties before filling in the Bindings.
Among other things, reading a declaration reads whether it's invalid,
then calling setInvalidDecl which assumes that all bindings of the
DecompositionDecl are available, but that isn't the case.
Deserialization should just set all properties directly without invoking
subsequent functions, so we just set the flag without using the setter.