In my earlier change to support noexcept in merge_exception_specifiers, 
I failed to consider the case of combining noexcept(false) with 
noexcept(true) in the context of implicitly declared member functions; 
that combination needs to result in noexcept(false), not noexcept(true).

While I was looking at this I also implemented DR 1073, which dropped 
compatibility between noexcept(false) and a dynamic-exception-specification.

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