swift234

[Driver / API Notes] Re-introduce "-apinotes" driver flag to compile API notes.

Using Clang to compile API notes introduces a dependency on the
underlying Clang, which can cause problems as Clang's support for API
notes evolves. Re-introduce the "-apinotes" driver flag into the Swift
driver to allow us to compile API notes, and use that (rather than
Clang) to precompile API notes for the overlays.