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

lib/Serialization/ModuleFile.cpp

@@ -268,7 +268,7 @@ static Optional<swift::LibraryKind> getActualLibraryKind(unsigned rawKind) {
 }
 
 
-ModuleFile::ModuleFile(llvm::OwningPtr<llvm::MemoryBuffer> &&input)
+ModuleFile::ModuleFile(std::unique_ptr<llvm::MemoryBuffer> input)
   : FileContext(nullptr),
     InputFile(std::move(input)),
     InputReader(reinterpret_cast<const uint8_t *>(InputFile->getBufferStart()),