[DebugInfo] Fix recursively generating debug info for same type
Debug Info generation already has a check to stop it from generating
debug info for a type with the same mangled name. However, most of the
code paths in debug info generation would not add the mangled name to
the cache while generation was not done. This patch fixes that so types
that are in-flight don't have their debug info generated twice.
rdar://142500619
