Instead of importing everything and filtering later (so all of clang modules get deserialized and associated Swift decls get created),
lazily import as Swift decls only the Clang decls that we need from a particular header.
This also fixes printing ObjC categories in the header as Swift extensions.
Swift SVN r28358
This translates clang arguments to swift ones, uses the bridging header
functionality to parse the provided file, and re-uses part of the module interface
printing implementation to print an interface for the header.
Part of rdar://19939192
Swift SVN r28062
