Sema:

 * queue full resolution of std.builtin.Type.Error when doing `@typeInfo`
   for error sets.

LLVM backend:

 * change a TODO comment to a proper explanation of why debug info
   for structs is left as a fwd decl sometimes.
 * remove handling of packed unions which does not match the type
   information or constant generation code.
 * remove copy+pasted code
 * fix union debug info not matching the memory layout
 * remove unnecessary error checks and type casting