swift234

[ClangImporter] Handle decls declared in imported headers.

This improves the -import-objc-header option to read decls from the header
as well. Any declaration that is not from a module will be considered to be
part of the "header module". Conversely, forward-declarations appearing in
the header will be resolved by looking through all modules responsible for
importing a header.

More of <rdar://problem/16702101>

Swift SVN r17492