As per #65930, the Clang importer's Clang instance may be configured with a different (higher) OS version than the compilation target itself in order to be able to load pre-compiled Clang modules that are aligned with the broader SDK, and match the SDK deployment target against which Swift modules are also built. Code-generation, however, must use the actual compilation target triple. This matches how Swift itself loads Swift module dependencies as well: dependency '.swiftinterface' files are type-checked against the availability epoch and code-generated against the actual compilation triple.
Resolves rdar://113712186
For example, when scanning a source module `Foo`, which, when depending on module `Bar` causes a cross-import overlay `_Foo_Bar` to be added, do not add this cross-import overlay when scanning `Foo` itself. For example, if `Foo` adds a dependency on `Bar` itself in its own dependency graph.
