ClangImporter synthesized declarations inherently do not have user written
code. Unfortunately, despite that they are not always marked implicit as they
should be. This was causing a crash when attempting to generate profile
coverage maps for synthesized constructors for imported structs.
I tried marking the constructors implicit, but that had too many knock-on
effects in tests. This more targeted fix unblocks compatibility suite testing
without trying to grapple with the implications of that more fundamental fix.
Resolves rdar://139486938.
