Attach location to constants

[gpetiot]

I still run into instances of "... and ... make inconsistent assumptions over interface Stdlib" when compiling even after cleaning thoroughly, will try on a clean repo later, the tests may be broken in the meantime.

edit: undrafting the PR, maybe someone will have an idea about what's causing this

[xavierleroy]

Could you please explain the motivation for this PR? Why is it a good idea to add locations to constants?

[gpetiot]

This is originally a change for ocamlformat (ocaml-ppx/ocamlformat#1749), someone suggested that I upstream this patch.

In ocamlformat we need to have the location of constants so we can re-attach the comments to the constants (and currently we have to read the literal source to retrieve the locations, whereas the parser can just provide them for free). Outside of ocamlformat, I don't know though.