Previously, the `src_node` field of `struct_decl`, `union_decl`,
`enum_decl`, and `opaque_decl` was optional, included in trailing data
only if a flag in `Small` was set. However, this was unnecessary logic:
AstGen always provided the source node. We can simplify a few bits of
logic by making this field non-optional, moving it into non-trailing
data.

There was one place where the field was actually omitted before: the
root struct of a file was at source node 0, so the node was
coincidentally elided. Therefore, this commit has a fixed cost of 4
bytes of ZIR per file.