swift234

[SourceKit][PrintObjcHeader] Avoid printing the same Swift decl twice.

Clang and Swift decls likely have many-to-one mappings; for instance,
a RecordDecl and a TypedefDecl in Clang are both imported as the same Swift
NominalTypeDecl. This commit reduces duplication when we print Clang decls in
Swift. rdar://23275959