Use extern type definitions in LLVM

After #1372 `extern type`s (and `extern interface`s) 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 / ...)