C++ PATCH to avoid ICE on unimplemented constexpr bits

[Jason Merrill]

We still need to implement references to the object under construction 
in constexpr constructors; this will also be useful for implementing the 
support for aggregate initialization of classes with NSDMIs.  But until 
that happens, we shouldn't crash.

Tested x86_64-pc-linux-gnu, applying to trunk.
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