4.4 C++ PATCH for c++/43024 (ice with -O2)

[Jason Merrill]

This is basically the same bug as 41183: cloning doesn't preserve 
cfun->language, so when mangling wants to do some template substitution, 
anything that relies on cfun->language being set will SEGV.  So we apply 
the same robustification fix.

Tested x86_64-pc-linux-gnu, applied to 4.4 and trunk.  I don't have a 
suitable reduced testcase to add to the testsuite; the one in the PR is 
large even after delta and doesn't crash on i686, just x86_64.  Volker, 
could I ask you to to take a shot at reducing it further?
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