Unify our diagnostic printing for errors of the form, "we didn't like

what we found when we looked into <blah>", where <blah> is a
DeclContext*. We can now format DeclContext*'s in nice ways, e.g.,
"namespace N", "the global namespace", "'class Foo'".

This is part of PR3990, but we're not quite there yet.

llvm-svn: 84028