C++ PATCH for c++/70344 (ICE with recursive constexpr)

[Jason Merrill]

cp_fold tries to fold calls to constexpr functions.  When we are inside 
a constexpr function at the time and see a call to the same function, 
this caused trouble because we hadn't adjusted the parameter types yet. 
  Fixed by handling this special case of calling a constexpr function 
that isn't defined yet.

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