debuginfo: nil and bot should not be described as uint in DWARF

[michaelwoerister]

Fixing this pretty much just amounts to changing the name from uint to () and !, respectively, in debuginfo::basic_type_metadata().

[mletterle]

I'm guessing the original author meant to have it as unit and not uint. I'm not sure if that's more appropriate or not than () and !, though it seems like it would be useful to distinguish between the two. I am kind of sad that DW_ATE_unsigned appears to be the only real good choice for the type encoding, a bit of research shows instances of implementations that use what appears to be a non-standard DW_ATE_void which may be more appropriate, but...