[clang] [llvm] [llvm][DebugInfo][clang] Finalize all declaration subprograms in DIBuilder::finalize() (PR #139914)

[Jeremy Morse via cfe-commits]

https://github.com/jmorse commented:

I suppose what this is encoding is a (subtle?) assumption about how definition-subprograms are modelled: they're created once  and are immutable. Wheras declaration-functions can be forwarded declared and then have extra information added to them when a definition is emitted. DIBuilder enforces this by refusing to let you re-set the retainedNodes of definition subprograms.

If there's no performance cost as a result of this patch, then it seems fine to go in to me., but is there a functional reason in a later patch that makes it necessary? Other folks might believe it's valuable to preserve the assumption about how debug-info is constructed within DIBuilder (CC @adrian-prantl @dwblaikie ). 

https://github.com/llvm/llvm-project/pull/139914