[dsymutil] Don't try to load Swift ASTs as objects.

With the threading refactoring, loading of object files happens before
checking whether we're dealing with a swift AST. While that's not an
issue per se, it causes a warning to be printed:

  warning: /path/to/a.swiftmodule: The file was not recognized as a valid object file
  note: while processing /path/to/a.swiftmodule

This suppresses the warning by checking for a Swift AST before
attempting to load is as an object file.

rdar://39240444

llvm-svn: 329553