C++ PATCH for c++/79640, infinite recursion with generic lambda

[Jason Merrill <jason at redhat dot com>]

Here, when we try to look up a constant variable from an enclosing
function when instantiating a generic lambda outside of the context of
that function, we make a dummy instantiation to use.  In this case,
the initializer refers to the variable, so we infinitely recurse until
we run out of stack.  Fixed by remembering the dummy instantiation
before we try to instantiate the initializer.

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

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