C++ PATCH for c++/52026 (use of enclosing constant var in lambda)

[Jason Merrill]

In this testcase, we see a use of a constant variable in a lambda 
(though the same thing can happen with a local class member function). 
We try to pull out the constant value in finish_id_expression, but 
because the initializer isn't a reduced constant yet, that fails and we 
end up just returning the variable.  At instantiation time tsubst 
instantiates the use of the variable by creating a new one in the lambda 
context, which is obviously broken.  A better solution is to defer 
deciding how to handle the variable until instantiation time, at which 
point we will know.  Since the logic for this is in 
finish_id_expression, it's easy to just return the name and look it up 
again at that point.

Tested x86_64-pc-linux-gnu, applying to trunk and 4.7.
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