[lldb] Support lazily named classes in the Objective-C classes

Generic classes in Swift have their name instantiated on request, since
the vast majority never need it, and it just wastes time and memory.
This results in LLDB being unable to determine the dynamic type of these
Swift objects.

The main issues is that lazily named classes are not added to the
gdb_objc_realized_classes hashtable. This means the class count in the
table doesn't change when a class is realized and LLDB doesn't know it
needs to re-parse the class info. But even if it did, the classes are
not in the hash table.

The first change in this patch is that we read
objc_debug_realized_class_generation_count and re-parse the class info
when the count changes.

The second change in this patch is that we use
objc_copyRealizedClassList (if available) to get all realized classes
from the runtime.

Unfortunately, objc_copyRealizedClassList calls _dyld_objc_class_count
in its implementation. As we know, the Objective-C parsing code might
get called before dyld is fully initialized, resulting in crashes or
even a stranded lock. Therefore we only use objc_copyRealizedClassList
when we know it's safe to do so by checking libSystemInitialized in
dyld_all_image_infos.

As a result, it's possible that the first time we read the Objective-C
runtime we are forced to use gdb_objc_realized_classes. This should be
fine, as there should be no lazily named classes at this point.
Subsequent queries will detect the change in realized class generation
count and use objc_copyRealizedClassList.

This patch keeps the old behavior when objc_copyRealizedClassList or
objc_debug_realized_class_generation_count are not available.

Differential revision: https://reviews.llvm.org/D99315