RFA: PATCH to ctor_for_folding for c++/61020 (ICE with typeid)

[Jason Merrill]

The C++ front end wants to be able to build up objects of the various 
typeinfo derived classes (such as __si_class_type_info) without needing 
to see the full definition of the class.  As part of this we build a 
VAR_DECL for the vtable, but never define it because we don't actually 
have declarations of the virtual functions, we're only using it for 
external references.  ctor_for_folding was assuming that all vtables are 
defined, which broke on this case.

Tested x86_64-pc-linux-gnu.  OK for trunk?
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