compiler: emit underlying constant in array_type length export

compiler: emit underlying constant in array_type length export

    In Array_type::do_export, when emitting a concrete array length,
    evaluate the length expression to an integer constant and emit that
    constant, instead of calling the more general method for emitting
    expressions. This is to avoid the possibility that we will need
    to emit a conversion, which could confuse the gccgoimporter.

    Fixes golang/go#30628.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/165741

From-SVN: r269443