C++ PATCH for c++/69889 (ICE with lambda conversion)

[Jason Merrill]

Here, we were crashing because the artificial thunk returned by the 
lambda conversion operator was dereferencing an invisible reference 
parameter instead of passing it through.  I needed to fix 
cp_genericize_r to avoid that like it already does for virtual function 
thunks.

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