compiler: allow @import of ZON without a result type

In particular, this allows importing `build.zig.zon` at comptime.

src/InternPool.zig

@@ -2136,6 +2136,7 @@ pub const Key = union(enum) {
         /// To avoid making this key overly complex, the type-specific data is hashed by Sema.
         reified: struct {
             /// A `reify`, `struct_init`, `struct_init_ref`, or `struct_init_anon` instruction.
+            /// Alternatively, this is `main_struct_inst` of a ZON file.
             zir_index: TrackedInst.Index,
             /// A hash of this type's attributes, fields, etc, generated by Sema.
             type_hash: u64,