DebugInfo: Improve type safety and simplify some subprogram finalization code

This probably ended up this way aften the subprogram<>function link
inversion and debug info metadata schema changes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@289697 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed