debuginfo: Make DWARF representation of enums uniform.

debuginfo: Make DWARF representation of enums uniform.

So far the DWARF information for enums was different
for regular enums, univariant enums, Option-like enums,
etc. Regular enums were encoded as unions of structs,
while the other variants were encoded as bare structs.

With the changes in this PR all enums are encoded as
unions so that debuggers can reconstruct if something
originally was a struct, a univariant enum, or an
Option-like enum. For the latter case, information
about the Null variant is encoded into the union field
name. This information can then be used by the
debugger to print a None value actually as None
instead of Some(0x0).