This addresses SR-15272.
When you have a function (e.g. 'foo') which is used in a constructor
initializer (e.g. constructor of some class 'Bar'), and you use the
constructor from swift code without directly using the function (e.g.
Bar's ctor is used from swift but foo isn't, foo is used from Bar's ctor),
you will get a link error. My fix is as follows:
When walking the clangAST to be imported, make sure you look at each
CXXCtorInitializer for each CXXConstructorDecl you see.
