[C++ Patch] Add maybe_constant_folded_value

[Jason Merrill]

On 11/13/2014 05:47 AM, Paolo Carlini wrote:
> shall we do something like the below?

Something similar, yes.  Though it would also be nice to avoid the 
redundant checking in the two functions: we only want to do constexpr 
folding if we did the tsubst.

Kai is working on folding changes, so I think let's let him weigh in on 
how this will interact with his work.

Jason