C++ PATCH for c++/48500 (C++0x crash on non-type-dependent but non-constant call in template)

[Jason Merrill]

constexpr fallout.  In fold_non_dependent_expr, we determined that the 
expression is not type-dependent, is potentially constant, and is not 
value-dependent, so we try to fold it.  But it isn't potentially 
constant.  So this patch fixes that determination.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.6 (regression).
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