Since commit b6367f723b (Swift/Ninja: Emit modules separately from
compilation, 2026-04-10) the .swiftmodule output moved from the
compile edge to a separate emit-module edge. The link edge does not
depend on the emit-module output, so ninja never runs the emit-module
edge when no other target in the build imports the module (e.g.
install-only targets).
Store the declared .swiftmodule output path during Swift object
statement generation and add it as an implicit dependency of the link
edge.
Issue: #27748
