swift234

Move the default implementation of ErrorType._domain into the standard library.

There is no reason for the compiler to be synthesizing a body of
_domain when it can be implemented in a protocol extension. As part of
this, fix a recent regression in the computed domain: it was using
string interpolation, which means that the recent changes not to print
qualified names affected the domain of the generated NSErrors. Oops.

Swift SVN r30343