These observer methods were used by external clients. Since we no longer
have the granularity between diagnostics and optimization, they're
rolled into a new observer callback, `performedSILProcessing`.
This (effectively) reverts commit 7b43e1d04d.
The dependency file that is being generated should not escape : and # in
the filename. This makes the behaviour of the filename escaping similar
to clang and GCC and fixes incorrect quoting of paths on Windows.
