[dsymutil] Fix missing member initializer.

This bug was found by the MSan bootstrap bot:
http://lab.llvm.org:8011/builders/sanitizer-x86_64-linux-bootstrap/builds/5330/steps/check-llvm%20msan/logs/stdio
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).