stage2 llvm: Use unpacked struct for unions and arrays

Our lowerings for various LLVM types assume that we can anticipate the
alignment/layout that LLVM will generate. Among other things, this
requires that we keep the alignment of our lowered LLVM types
synchronized with their expected alignment in Zig.

 - Arrays were using packed struct types, which is seems to be
   incorrect since array elements are supposed to be self-aligned.

 - Unions were using packed struct types for their payload, which causes
   layout divergence between what stage2 expects and what LLVM generates

Consider this lowered union type:
```llvm
%Value = type { <{ i64, [8 x i8] }>, i1, [7 x i8] } ; 24 bytes, align(1)
%ErrorUnion = type { %Value, i16 } ; 26 bytes, align(2)
```

Zig expects Value to be align(8) and, by extension, for ErrorUnion to be
size 32.

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