IR: Handle self-referencing DICompositeTypes in DIBuilder
Add API to DIBuilder to handle self-referencing `DICompositeType`s.
Self-references aren't expected in the debug info graph, and we take
advantage of that by only calling `resolveCycles()` on nodes that were
once forward declarations (otherwise, DIBuilder needs an expensive
tracking reference to every unresolved node it creates, which in cyclic
graphs is *all of them*).
However, clang seems to create self-referencing `DICompositeType`s. Add
API to manage this safely. The paired commit to clang will include the
regression test.
I'll make the `DICompositeType` API `private` in a follow-up to prevent
misuse (I've separated that to prevent build failures from missing the
clang commit).
llvm-svn: 224482