C++ PATCH for c++/85149, generic lambda and constexpr if

[Jason Merrill]

Now that we partially instantiate generic lambdas, we run into an
issue with C++17 constexpr if: in the testcase, the branches of the
constexpr if become non-dependent when partially instantiated, but the
condition remains dependent.  In that situation, we mustn't substitute
into the branches, so with this patch I use the
PACK_EXPANSION_EXTRA_ARGS mechanism for constexpr if as well.

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