Remove unused diagnostic and diagnostic group.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211464 91177308-0d34-0410-b5e6-96231b3b80d8
Revert r211402 (and r211408,r211410), "CodeGen: Refactor dynamic_cast and typeid" It crashes msvc codegen in clang/test/SemaCXX/undefined-internal.cpp.

It is reproducible with:

  $ clang -cc1 -triple i686-win32 -emit-llvm-only clang/test/SemaCXX/undefined-internal.cpp

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211467 91177308-0d34-0410-b5e6-96231b3b80d8