C backend: emit decls sorted by dependencies

The C backend is the only backend that requires each decl to be output
in an order that satisfies the dependency graph. Here it is implemented
with a simple algorithm based on a `remaining_decls` set, using the
`dependencies` edges that are already stored for each Decl.

This satisfies incremental compilation as well as how `zig test` works,
which calls `updateDecl` on `test_functions`.

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