Use extern type definitions in LLVM

[marzipankaiser]

After #1372 extern types (and extern interfaces) can have right-hand sides.

We should use this in the llvm backend to allow something like

extern type Int64 =
  llvm "i64, 64, no eraser, no sharer"

For this, we need a way to specify all the necessary parts (type, size, eraser, sharer, ...), there are mostly two options:

1. have special syntax in the extern string (like above)
2. have syntax parsed in the Effekt parser (e.g. allow multiple extern strings / tuples / ...)