RFC: PATCH to allow passing non-trivial types through ...

[Jason Merrill]

A customer was recently complaining about G++ rejecting code that tries 
to pass a type with a non-trivial copy constructor through ..., which is 
undefined in C++98 and conditionally-supported in C++11.  In GCC 3.1 and 
below we gave a warning and then did a bitwise copy.  From GCC 3.2 to 
4.4 we gave a warning and then aborted at runtime.  From 4.5 on we have 
just given an error.

On considering the request, it occurred to me that we could handle 
variadic arguments of non-trivial types the same way we handle normal 
value arguments of such types: pass by invisible reference.  So this 
patch implements that.  Since it's been so long since this was allowed 
at all, I don't think we need to worry about ABI incompatibility with 
the 3.1 behavior.

Thoughts?
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