C++ PATCH for c++/48936 (failure with static constant template variable in sizeof)

[Jason Merrill]

This issue had to do with us bailing out too soon when trying to 
mark_used a constant variable in an unevaluated context.  I fixed this 
for 4.6 as part of the constexpr work, but a subset of that patch seems 
suitable for backporting.

Tested x86_64-pc-linux-gnu, applying to 4.4 and 4.5.
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