LangRef: Removed some outdated text about DIDerivedType

This text is also incorrect (much like r266540).  It looks like I missed
updating some of what I moved from SourceLevelDebugging.rst in r232566.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266544 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed