C++ PATCH to avoid double copying of default arguments

[Jason Merrill]

While working on something else I noticed that we were copying the trees 
in a default argument twice: once in break_out_target_exprs and then 
again with an explicit unshare_expr.  So I removed the redundant 
unshare_expr.  This didn't cause any regressions by itself, but just to 
be safe I also changed bot_manip to copy everything.

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