Remove buggy example code from the documentation

Instead point readers to the latest, correct example code in SVN until we find
a way to automatically include example sources into the documentation (or until
someone steps up to maintain these actively).

This ensures that the examples are up-to-date, buildable, and most of all that
readers don't pick up incorrect usage.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200125 91177308-0d34-0410-b5e6-96231b3b80d8
Identify two more unsafe uses of getCustomDiagID()

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200126 91177308-0d34-0410-b5e6-96231b3b80d8