swift234

Accept swiftmodule files on the command line as partial AST inputs.

Each loaded file gets added to the main module, rather than being a standalone
separate module. In theory, this will be used to assemble several partial
ASTs into a complete module. In practice, there's still a ways to go...but
this can already round-trip a single module file.

This also factors out the FileUnit-creating part of SerializedModuleLoader,
which should help clients like SourceKit that don't need to search for a
swiftmodule file associated with a particular import.

Swift SVN r10952