Add incremental Swift static lib build test

Ensure that we're actually trying to rebuild libB when the public
interface for libA changes. Without handling the swiftmodule dependency
edge correctly, we would only get a linker error because libA didn't
have the symbol that libB depended on. With the fix, we get a proper
compiler error because ninja knows to rebuild the intermediate libB
when the public interface of libA changes. This is more actionable.
5 files changed