compiler: make xx_constant_value methods non-const

This changes the Expression {numeric,string,boolean}_constant_value
methods 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.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/536641