DebugInfo: Add an assert regarding the subprogram in the subprogram map matching the abstract subprogram.

I'm not sure this is how it'll be going forward (I'd rather prefer the
definition to be in the main SP mapping, for various reasons) but this
helps me understand how it is today.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209009 91177308-0d34-0410-b5e6-96231b3b80d8