Revert "Rename LLVMContext diagnostic handler types and functions."

This reverts my commit 171047. Now that I've removed my misguided attempt to
support backend warnings, these diagnostics are only about inline assembly.
It would take quite a bit more work to generalize them properly, so I'm
just reverting this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174860 91177308-0d34-0410-b5e6-96231b3b80d8