[CGDebugInfo] Finalize SubPrograms when we're done with them

`GenerateVarArgsThunk` in `CGVTables` clones a function before the frontend
is done emitting the compilation unit. Because of the way that DIBuilder
works, this means that the attached subprogram had incomplete (temporary)
metadata. Cloning such metadata is semantically disallowed, but happened
to work anyway due to bugs in the cloning logic. rL304226 attempted to fix
up that logic, but in the process exposed the incorrect API use here and
had to be reverted. To be able to fix this, I added a new method to
DIBuilder in rL304467, to allow finalizing a subprogram independently
of the entire compilation unit. Use that here, in preparation of re-applying
rL304226.

Reviewers: aprantl, dblaikie
Differential Revision: https://reviews.llvm.org/D33705

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@304470 91177308-0d34-0410-b5e6-96231b3b80d8