swift234

[serialization] Add a special encoding for the imported header module.

This allows a loaded swiftmodule to refer to decls that come from a header
imported by the module itself. There's an unfortunate sharing of the
namespace here with any /other/ headers loaded by /other/ modules (including
the module currently being compiled), but that shouldn't be too much of an
issue in practice -- the declarations would need to be compatible anyway if
you were doing the same thing in Objective-C.

More of <rdar://problem/16702101>. This is possibly the end of the compiler
work, but I'd like to add more tests to make sure everything is working.

Swift SVN r17639