We do iterate into extern C declarations when building the Swift lookup table during PCH generation.
0fad799f51/lib/ClangImporter/SwiftLookupTable.cpp (L2140-L2146)
However, we don’t import `extern "C"` declarations while parsing the bridging header (eg. when no `-pch-output-dir` is specified during code completion). This caused us to miss functions annotated as `extern "C"` in code completion.
rdar://127512985