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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@84028 91177308-0d34-0410-b5e6-96231b3b80d8