[v3] Fix debug-mode std::bitset vs constexpr

[Paolo Carlini]

Hi,

testing debug-mode revealed a pretty serious issue in the form of two
compile-time fails: since _Safe_sequence_base has a non
trivial-destructor (and possibly for other reasons too, I didn't
investigate in detail yet), deriving from it caused std::bitset to not
be a literal type anymore. For now I'm just suppressing the derivation,
thus disabling some debug-mode checks in C++0x mode (to be honest there
aren't so many, anyway), better solutions welcome of course.

Tested x86_64-linux, committed to mainline.

Paolo.

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