This patch to the Go frontend changes the Expression
{numeric,string,boolean}_constant_value methods to be non-const.  This
does not affect anything immediately, but will be useful for later CLs
in this series.

The only real effect is to Builtin_call_expression::do_export, which
remains const and can no longer call numeric_constant_value.  But it
never needed to call it, as do_export runs after do_lower, and
do_lower replaces a constant expression with the actual constant.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian