This patch is a prerequisite of a forthcoming patch that improves the types used to represent symbols (names of statically-allocated entities) throughout the middle end and backend. This in turn is needed for the Asm_directives layer on which the DWARF support is based.

I think a good general principle, when writing a pass that operates on some particular IR, is to avoid generating constructs from previous IRs and sending them back through part of the current pass. Instead, it is better to refactor the current pass so that the constructs you want can be generated directly in your current language.

This patch changes Cmmgen to follow this principle for constants. The reason this is necessary is because, with the forthcoming patch, it is not always possible to construct a value of the type that describes middle-end symbols (such as will appear in Clambda) during Cmmgen. I would rather not get into the details here, but in summary, this relates to the fact that during Cmmgen we may be generating code (e.g. for a startup file) that does not stem from an OCaml compilation unit.

There are a couple of other related changes that I thought reasonable to include with this patch, since our workflow management becomes difficult if there are just too many pull requests. One of them passes the list of constants, for the classic Closure mode, through from Asmgen rather than reading the Compilenv global state in Cmmgen; this is consistent with what Flambda does. The other pulls the global state from Cmmgen, much of which is to do with constants, into a separate file. Warning 40 is suppressed to increase code legibility (e.g. with Local and Global) and reduce proliferation of "open".

@lthls Could you please look at this? (Some amount of this is just moving code upwards in cmmgen.ml; you might want to diff that separately.)