Clients of the dependency scanning service may need to add dependencies
based on the visibility of importing modules, for example, when
determining whether a Swift overlay dependency should be brought in
based on whether there's a corresponding **visible** clang module for
it.
This patch introduces a new field `VisibleModules` that contains all the
visible top-level modules in a given TU.
Because visibility is determined by which headers or (sub)modules were
imported, and not top-level module dependencies, the scanner now
performs a separate DFS starting from what was directly imported for
this computation.
In my local performance testing, there was no observable performance
impact.
resolves: rdar://151416358
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Co-authored-by: Jan Svoboda <jan@svoboda.ai>
15c3793cdf