Don't instantiate lambda closure types in default member initializers

when instantiating the enclosing class.

We'll build new lambda closure types if and when we instantiate the
default member initializer, and instantiating the closure type by itself
can go wrong in cases where we fully-instantiate nested classes (in
explicit instantiations of the enclosing class and when the enclosing
class is a local class) -- we will instantiate the 'operator()' as a
regular function rather than as a lambda call operator, so it doesn't
get to use its captures, has the wrong 'this' type, etc.