compiler: handle int-to-string conversion with large integer constant

compiler: handle int-to-string conversion with large integer constant

    Currently, Type_conversion_expression::do_is_constant thinks the
    int-to-string conversion is constant if the integer operand is
    constant, but Type_conversion_expression::do_get_backend actually
    generates a call to runtime.intstring if the integer does not fit
    in a "ushort", which makes it not suitable in constant context,
    such as static initializer.

    This CL makes it handle all constant integer input as constant,
    generating constant string.

    Fixes golang/go#32347.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/179777

From-SVN: r271821