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
