swift234

Allow diagnostics to accept declarations in lieu of source locations.

Swift's diagnostic system is built on the quaint notion that every
declaration known to the front end has a valid source location to
which diagnostics mentioning that declaration (say, in a "here is a
candidate" note) can point. However, with a real module system, the
source corresponding to a declaration in a module does not need to be
present, so we can't rely on source locations.

Instead of source locations, allow diagnostics to be anchored at a
declaration. If that declaration has source-location information, it
is used. Otherwise, we just drop source-location information for
now. In the future, we'll find a better way to render this information
so it feels natural for the programmer.


Swift SVN r3413