[lldb] Factor out iteration over runtime types from GetChildCompilerTypeAtIndex()

This patch introduces a new class SwiftRuntimeTypeVisitor that exists
to unify iteration over runtime type information. The visitor callback
has closure parameters that can be called to make additional expensive
queries on a child.

TODO: This is not the final evolution step.

- We probably should remove the "depth" parameter entirely and implement the access path computation for GetIndexOfChildMemberWithName at a different layer.

- We could cache the results for the last execution context.

Relanding with an off-by-one error fixed in
GetExistentialSyntheticChildren that was caught by ASAN!

Author: adrian-prantl

lldb/source/Plugins/LanguageRuntime/Swift/SwiftLanguageRuntime.h

@@ -67,6 +67,8 @@ struct SuperClassType;
 using ThreadSafeReflectionContext = LockGuarded<ReflectionContextInterface>;
 
 class SwiftLanguageRuntime : public LanguageRuntime {
+  friend class SwiftRuntimeTypeVisitor;
+
 protected:
   SwiftLanguageRuntime(Process &process);