Initial support for emitting DWARF for static vars.

Only supports crate level statics. No debug info is generated for
function level statics. Closes #9227.
auto merge of #13143 : gentlefolk/rust/issue-9227, r=michaelwoerister

Only supports crate level statics. No debug info is generated for function level statics. Closes #9227.

As discussed at the end of the comments for #9227, I took an initial stab at adding support for function level statics and decided it would be enough work to warrant being split into a separate issue.

See #13144 for the new issue describing the need to add support for function level static variables.