C++ PATCH for c++/48003 (allow integer overflow in template args with -fpermissive)

[Jason Merrill]

This patch changes convert_nontype_argument to defer to 
cxx_constant_value on whether or not to go ahead with a semi-constant 
template argument, and whether or not to give a diagnostic.  Making this 
change revealed a bug in potential_constant_expression_1, which was 
failing to check for overflow.

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